Add target maintainer information for mips64-unknown-linux-muslabi64

The mips64-unknown-linux-muslabi64 target is currently rather broken,
but I'm working on getting it fixed so that it can at least be used
again.

While I can't commit to maintaining the LLVM side of this target, I
don't mind looking into any other MIPS or musl-related issues that arise
with this target.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

src/doc/rustc/src/SUMMARY.md

@@ -83,6 +83,7 @@
     - [m68k-unknown-linux-gnu](platform-support/m68k-unknown-linux-gnu.md)
     - [m68k-unknown-none-elf](platform-support/m68k-unknown-none-elf.md)
     - [mips64-openwrt-linux-musl](platform-support/mips64-openwrt-linux-musl.md)
+    - [mips64-unknown-linux-muslabi64](platform-support/mips64-unknown-linux-muslabi64.md)
     - [mipsel-sony-psx](platform-support/mipsel-sony-psx.md)
     - [mipsel-unknown-linux-gnu](platform-support/mipsel-unknown-linux-gnu.md)
     - [mips\*-mti-none-elf](platform-support/mips-mti-none-elf.md)

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

@@ -333,7 +333,7 @@ target | std | host | notes
 `mips-unknown-linux-uclibc` | ✓ |  | MIPS Linux with uClibc
 [`mips64-openwrt-linux-musl`](platform-support/mips64-openwrt-linux-musl.md) | ? |  | MIPS64 for OpenWrt Linux musl 1.2.3
 `mips64-unknown-linux-gnuabi64` | ✓ | ✓ | MIPS64 Linux, N64 ABI (kernel 4.4, glibc 2.23)
-`mips64-unknown-linux-muslabi64` | ✓ |  | MIPS64 Linux, N64 ABI, musl 1.2.3
+[`mips64-unknown-linux-muslabi64`](platform-support/mips64-unknown-linux-muslabi64.md) | ✓ | ✓ | MIPS64 Linux, N64 ABI, musl 1.2.3
 `mips64el-unknown-linux-gnuabi64` | ✓ | ✓ | MIPS64 (little endian) Linux, N64 ABI (kernel 4.4, glibc 2.23)
 `mips64el-unknown-linux-muslabi64` | ✓ |  | MIPS64 (little endian) Linux, N64 ABI, musl 1.2.3
 `mipsel-sony-psp` | * |  | MIPS (LE) Sony PlayStation Portable (PSP)

src/doc/rustc/src/platform-support/mips64-unknown-linux-muslabi64.md

@@ -0,0 +1,49 @@
+# mips64-unknown-linux-muslabi64
+
+**Tier: 3**
+
+Target for 64-bit big endian MIPS Linux programs using musl libc and the N64 ABI.
+
+## Target maintainers
+
+[@Gelbpunkt](https://github.com/Gelbpunkt)
+
+## Requirements
+
+Building the target itself requires a 64-bit big endian MIPS 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 = ["mips64-unknown-linux-muslabi64"]
+```
+
+Make sure your C compiler is included in `$PATH`, then add it to the
+`bootstrap.toml`:
+
+```toml
+[target.mips64-unknown-linux-muslabi64]
+cc = "mips64-linux-musl-gcc"
+cxx = "mips64-linux-musl-g++"
+ar = "mips64-linux-musl-ar"
+linker = "mips64-linux-musl-gcc"
+```
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will first need to build Rust with the target enabled (see
+"Building the target" above).
+
+## Cross-compilation
+
+This target can be cross-compiled from any host.
+
+## Testing
+
+This target can be tested as normal with `x.py` on a 64-bit big endian MIPS
+host or via QEMU emulation.