MIPS: Layout refactor

MIPS32 and MIPS64 userspaces are mostly the same save the obvious ABI
difference, hence most of the const definitions could be shared.

This commit is not introducing any new significant additions, but mostly
just shuffles the existing files around to make the directory structure
more similar to the other, more mature platforms. The structs and extern
fn definitions are not touched. Plus a couple of new consts for MIPS64
to match MIPS32 coverage:

* `RLIM_INFINITY`
* `SYS_gettid`

Passed `libc-test` on Gentoo MIPS64el (n64 ABI), on a Loongson 3A2000
box. The test suite itself needs some more love to actually compile
cleanly on MIPS (`ctest` transitively depends on ancient `libc` version
which doesn't build on MIPS), but let's save that for another day.