Add riscv32 and riscv64 musl to supported platform targets

Signed-off-by: Khem Raj <[email protected]>

Author: kraj

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

@@ -204,7 +204,9 @@ target | std | host | notes
 `powerpc64-unknown-linux-musl` | ? |  |
 `powerpc64-wrs-vxworks` | ? |  |
 `powerpc64le-unknown-linux-musl` | ? |  |
+`riscv64gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 4.20, musl 1.2.0)
 `riscv32gc-unknown-linux-gnu` |   |   | RISC-V Linux (kernel 5.4, glibc 2.33)
+`riscv32gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 5.4, musl + RISCV32 support patches)
 `sparc-unknown-linux-gnu` | ✓ |  | 32-bit SPARC Linux
 `sparc64-unknown-netbsd` | ✓ | ✓ | NetBSD/sparc64
 `sparc64-unknown-openbsd` | ? |  |