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/api/Cargo.toml

@@ -21,13 +21,16 @@ sp-api-proc-macro = { path = "proc-macro", default-features = false }
 sp-core = { path = "../core", default-features = false }
 sp-std = { path = "../std", default-features = false }
 sp-runtime = { path = "../runtime", default-features = false }
+sp-runtime-interface = { path = "../runtime-interface", default-features = false }
 sp-externalities = { path = "../externalities", default-features = false, optional = true }
 sp-version = { path = "../version", default-features = false }
 sp-state-machine = { path = "../state-machine", default-features = false, optional = true }
 sp-trie = { path = "../trie", default-features = false, optional = true }
 hash-db = { version = "0.16.0", optional = true }
 thiserror = { version = "1.0.48", optional = true }
-scale-info = { version = "2.10.0", default-features = false, features = ["derive"] }
+scale-info = { version = "2.10.0", default-features = false, features = [
+	"derive",
+] }
 sp-metadata-ir = { path = "../metadata-ir", default-features = false, optional = true }
 log = { version = "0.4.17", default-features = false }
 
@@ -46,6 +49,7 @@ std = [
 	"sp-externalities",
 	"sp-externalities?/std",
 	"sp-metadata-ir?/std",
+	"sp-runtime-interface/std",
 	"sp-runtime/std",
 	"sp-state-machine/std",
 	"sp-std/std",