Auto merge of #37625 - xen0n:rustbuild-mips, r=alexcrichton
Auto merge of #37625 - xen0n:rustbuild-mips, r=alexcrichton

rustbuild: support MIPS host builds

There is a *little* code duplication, but primarily for sake of "match exhaustiveness". Let's blame Linux/MIPS for not exposing endianness explicitly in `uname -m` (that's user-space interface and as such is frozen).

Currently the build won't work as we have to wait for a new stage0 for the MIPS host compilers, but this paves the way to self-hosted Rust on MIPS. The cross-compiled MIPS binaries are confirmed to work on the Loongson 3A2000 (MIPS64r2-compatible) so we have plenty of confidence that they'll work on other MIPS platforms too, as Linux/MIPS user-space ABI is consistent across machines of the same bitness.

r? @alexcrichton

aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust