[mips64-openwrt-musl] Tier 3 tuple for Mips64 OpenWrt

This incorporates rust-lang into the OpenWrt build system for
Mips64 targets.

Signed-off-by: Donald Hoskins <grommish@gmail.com>

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

@@ -244,6 +244,7 @@ target | std | host | notes
 `i686-wrs-vxworks` | ? |  |
 `m68k-unknown-linux-gnu` | ? |  | Motorola 680x0 Linux
 `mips-unknown-linux-uclibc` | ✓ |  | MIPS Linux with uClibc
+`mips64-openwrt-linux-musl` | ? |  | MIPS64 for OpenWrt Linux MUSL
 `mipsel-sony-psp` | * |  | MIPS (LE) Sony PlayStation Portable (PSP)
 `mipsel-unknown-linux-uclibc` | ✓ |  | MIPS (LE) Linux with uClibc
 `mipsel-unknown-none` | * |  | Bare MIPS (LE) softfloat

src/doc/rustc/src/platform-support/mips64-openwrt-linux-musl.md

@@ -0,0 +1,28 @@
+# `mips64-openwrt-linux-musl`
+**Tier: 3**
+
+## Target maintainers
+- Donald Hoskins `grommish@gmail.com`, https://github.com/Itus-Shield
+
+## Requirements
+This target is cross-compiled. There is no support for `std`. There is no
+default allocator, but it's possible to use `alloc` by supplying an allocator.
+
+By default, Rust code generated for this target uses `-msoft-float` and is
+dynamically linked.
+
+This target generated binaries in the ELF format.
+
+## Building the target
+This target is built exclusively within the `OpenWrt` build system via
+the `rust-lang` HOST package
+
+## Building Rust programs
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will either need to build Rust with the target enabled (see
+"Building the target" above).
+
+## Testing
+As `mips64-openwrt-linux-musl` supports a variety of different environments and does
+not support `std`, this target does not support running the Rust testsuite at this
+time.