Initial support for building RISC-V runtimes targeting PolkaVM (#3179)

This PR adds initial support for building RISC-V runtimes targeting
PolkaVM.

- Setting the `SUBSTRATE_RUNTIME_TARGET=riscv` environment variable will
now build a RISC-V runtime instead of a WASM runtime.
- This only adds support for *building* runtimes; running them will need
a PolkaVM-based executor, which I will add in a future PR.
- Only building the minimal runtime is supported (building the Polkadot
runtime doesn't work *yet* due to one of the dependencies).
- The builder now sets a `substrate_runtime` cfg flag when building the
runtimes, with the idea being that instead of doing `#[cfg(not(feature =
"std"))]` or `#[cfg(target_arch = "wasm32")]` to detect that we're
building a runtime you'll do `#[cfg(substrate_runtime)]`. (Switching the
whole codebase to use this will be done in a future PR; I deliberately
didn't do this here to keep this PR minimal and reviewable.)
- Further renaming of things (e.g. types, environment variables and proc
macro attributes having "wasm" in their name) to be target-agnostic will
also be done in a future refactoring PR (while keeping backwards
compatibility where it makes sense; I don't intend to break anyone's
workflow or create unnecessary churn).
- This PR also fixes two bugs in the `wasm-builder` crate:
* The `RUSTC` environment variable is now removed when invoking the
compiler. This prevents the toolchain version from being overridden when
called from a `build.rs` script.
* When parsing the `rustup toolchain list` output the `(default)` is now
properly stripped and not treated as part of the version.
- I've also added a minimal CI job that makes sure this doesn't break in
the future. (cc @paritytech/ci)

cc @athei

------

Also, just a fun little tidbit: quickly comparing the size of the built
runtimes it seems that the PolkaVM runtime is slightly smaller than the
WASM one. (`production` build, with the `names` section substracted from
the WASM's size to keep things fair, since for the PolkaVM runtime we're
currently stripping out everything)

- `.wasm`: 625505 bytes
- `.wasm` (after wasm-opt -O3): 563205 bytes
- `.wasm` (after wasm-opt -Os): 562987 bytes
- `.wasm` (after wasm-opt -Oz): 536852 bytes
- `.polkavm`: ~~580338 bytes~~ 550476 bytes (after enabling extra target
features; I'll add those in another PR once we have an executor working)

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/primitives/io/src/lib.rs

@@ -1737,20 +1737,20 @@ mod tracing_setup {
 
 pub use tracing_setup::init_tracing;
 
-/// Allocator used by Substrate when executing the Wasm runtime.
-#[cfg(all(target_arch = "wasm32", not(feature = "std")))]
-struct WasmAllocator;
+/// Allocator used by Substrate from within the runtime.
+#[cfg(substrate_runtime)]
+struct RuntimeAllocator;
 
-#[cfg(all(target_arch = "wasm32", not(feature = "disable_allocator"), not(feature = "std")))]
+#[cfg(all(not(feature = "disable_allocator"), substrate_runtime))]
 #[global_allocator]
-static ALLOCATOR: WasmAllocator = WasmAllocator;
+static ALLOCATOR: RuntimeAllocator = RuntimeAllocator;
 
-#[cfg(all(target_arch = "wasm32", not(feature = "std")))]
+#[cfg(substrate_runtime)]
 mod allocator_impl {
 	use super::*;
 	use core::alloc::{GlobalAlloc, Layout};
 
-	unsafe impl GlobalAlloc for WasmAllocator {
+	unsafe impl GlobalAlloc for RuntimeAllocator {
 		unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
 			allocator::malloc(layout.size() as u32)
 		}
@@ -1761,8 +1761,27 @@ mod allocator_impl {
 	}
 }
 
-/// A default panic handler for WASM environment.
-#[cfg(all(not(feature = "disable_panic_handler"), not(feature = "std")))]
+/// Crashes the execution of the program.
+///
+/// Equivalent to the WASM `unreachable` instruction, RISC-V `unimp` instruction,
+/// or just the `unreachable!()` macro everywhere else.
+pub fn unreachable() -> ! {
+	#[cfg(target_family = "wasm")]
+	{
+		core::arch::wasm32::unreachable();
+	}
+
+	#[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
+	unsafe {
+		core::arch::asm!("unimp", options(noreturn));
+	}
+
+	#[cfg(not(any(target_arch = "riscv32", target_arch = "riscv64", target_family = "wasm")))]
+	unreachable!();
+}
+
+/// A default panic handler for the runtime environment.
+#[cfg(all(not(feature = "disable_panic_handler"), substrate_runtime))]
 #[panic_handler]
 #[no_mangle]
 pub fn panic(info: &core::panic::PanicInfo) -> ! {
@@ -1774,11 +1793,11 @@ pub fn panic(info: &core::panic::PanicInfo) -> ! {
 	#[cfg(not(feature = "improved_panic_error_reporting"))]
 	{
 		logging::log(LogLevel::Error, "runtime", message.as_bytes());
-		core::arch::wasm32::unreachable();
+		unreachable();
 	}
 }
 
-/// A default OOM handler for WASM environment.
+/// A default OOM handler for the runtime environment.
 #[cfg(all(not(feature = "disable_oom"), enable_alloc_error_handler))]
 #[alloc_error_handler]
 pub fn oom(_: core::alloc::Layout) -> ! {
@@ -1789,7 +1808,7 @@ pub fn oom(_: core::alloc::Layout) -> ! {
 	#[cfg(not(feature = "improved_panic_error_reporting"))]
 	{
 		logging::log(LogLevel::Error, "runtime", b"Runtime memory exhausted. Aborting");
-		core::arch::wasm32::unreachable();
+		unreachable();
 	}
 }