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/runtime-interface/proc-macro/src/runtime_interface/bare_function_interface.rs

@@ -109,7 +109,12 @@ fn function_no_std_impl(
 	};
 	let maybe_unreachable = if method.should_trap_on_return() {
 		quote! {
-			; core::arch::wasm32::unreachable();
+			;
+			#[cfg(target_family = "wasm")]
+			{ core::arch::wasm32::unreachable(); }
+
+			#[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
+			unsafe { core::arch::asm!("unimp", options(noreturn)); }
 		}
 	} else {
 		quote! {}
@@ -118,7 +123,7 @@ fn function_no_std_impl(
 	let attrs = method.attrs.iter().filter(|a| !a.path().is_ident("version"));
 
 	let cfg_wasm_only = if is_wasm_only {
-		quote! { #[cfg(target_arch = "wasm32")] }
+		quote! { #[cfg(substrate_runtime)] }
 	} else {
 		quote! {}
 	};

substrate/primitives/runtime-interface/proc-macro/src/runtime_interface/host_function_interface.rs

@@ -116,8 +116,8 @@ fn generate_extern_host_function(
 		#(#cfg_attrs)*
 		#[doc = #doc_string]
 		pub fn #function ( #( #args ),* ) #return_value {
+			#[cfg_attr(any(target_arch = "riscv32", target_arch = "riscv64"), #crate_::polkavm::polkavm_import(abi = #crate_::polkavm::polkavm_abi))]
 			extern "C" {
-				/// The extern function.
 				pub fn #ext_function (
 					#( #arg_names: <#arg_types as #crate_::RIType>::FFIType ),*
 				) #ffi_return_value;