Add platform support document for riscv64gc-unknown-linux-musl

Author: Amanieu

src/doc/rustc/src/SUMMARY.md

@@ -65,6 +65,7 @@
     - [riscv32im-risc0-zkvm-elf](platform-support/riscv32im-risc0-zkvm-elf.md)
     - [riscv32imac-unknown-xous-elf](platform-support/riscv32imac-unknown-xous-elf.md)
     - [riscv32*-unknown-none-elf](platform-support/riscv32-unknown-none-elf.md)
+    - [riscv64gc-unknown-linux-musl](platform-support/riscv64gc-unknown-linux-musl.md)
     - [sparc-unknown-none-elf](./platform-support/sparc-unknown-none-elf.md)
     - [*-pc-windows-gnullvm](platform-support/pc-windows-gnullvm.md)
     - [\*-nto-qnx-\*](platform-support/nto-qnx.md)

src/doc/rustc/src/platform-support.md

@@ -97,6 +97,7 @@ target | notes
 `powerpc64-unknown-linux-gnu` | PPC64 Linux (kernel 3.2, glibc 2.17)
 `powerpc64le-unknown-linux-gnu` | PPC64LE Linux (kernel 3.10, glibc 2.17)
 `riscv64gc-unknown-linux-gnu` | RISC-V Linux (kernel 4.20, glibc 2.29)
+`riscv64gc-unknown-linux-musl` | RISC-V Linux (kernel 4.20, musl 1.2.3)
 `s390x-unknown-linux-gnu` | S390x Linux (kernel 3.2, glibc 2.17)
 `x86_64-unknown-freebsd` | 64-bit FreeBSD
 `x86_64-unknown-illumos` | illumos
@@ -352,7 +353,6 @@ target | std | host | notes
 [`riscv64gc-unknown-hermit`](platform-support/hermit.md) | ✓ |   | RISC-V Hermit
 `riscv64gc-unknown-freebsd` |   |   | RISC-V FreeBSD
 `riscv64gc-unknown-fuchsia` |   |   | RISC-V Fuchsia
-`riscv64gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 4.20, musl 1.2.3)
 [`riscv64gc-unknown-netbsd`](platform-support/netbsd.md) | ✓ | ✓ | RISC-V NetBSD
 [`riscv64gc-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | OpenBSD/riscv64
 [`riscv64-linux-android`](platform-support/android.md) |   |   | RISC-V 64-bit Android

src/doc/rustc/src/platform-support/riscv64gc-unknown-linux-musl.md

@@ -0,0 +1,47 @@
+# riscv64gc-unknown-linux-musl
+
+**Tier: 2**
+
+Target for RISC-V Linux programs using musl libc.
+
+## Target maintainers
+
+- [@Amanieu](https://github.com/Amanieu)
+- [@kraj](https://github.com/kraj)
+
+## Requirements
+
+Building the target itself requires a RISC-V compiler that is supported by `cc-rs`.
+
+## Building the target
+
+The target can be built by enabling it for a `rustc` build.
+
+```toml
+[build]
+target = ["riscv64gc-unknown-linux-musl"]
+```
+
+Make sure your C compiler is included in `$PATH`, then add it to the `config.toml`:
+
+```toml
+[target.riscv64gc-unknown-linux-musl]
+cc = "riscv64-linux-gnu-gcc"
+cxx = "riscv64-linux-gnu-g++"
+ar = "riscv64-linux-gnu-ar"
+linker = "riscv64-linux-gnu-gcc"
+```
+
+## Building Rust programs
+
+This target are distributed through `rustup`, and otherwise require no
+special configuration.
+
+## Cross-compilation
+
+This target can be cross-compiled from any host.
+
+## Testing
+
+This target can be tested as normal with `x.py` on a RISC-V host or via QEMU
+emulation.