These aren't exposed on non-MIPS musl targets, but since they're
part of a kernel API, they're still applicable, so we just don't
test them there but expose them anyways.

Note that termios2 doesn't exist on powerpc(64), termios
is used instead.

Signed-off-by: Tom Kirchner <tjk@amazon.com>
Signed-off-by: Ben Cressey <bcressey@amazon.com>

In addition, move the syscalls to the mips module file because they are
all identical in mips32 and mips64.

As a result of this commit, glob64_t will be added to mips64.

MINSIGSTKSZ is a minimum size of process stack allowed

Having the B* constants in `unix/notbsd/mod.rs` passed CI tests
except for powerpc.  So we'll try moving into individual
arch/ABI that the CI tests cover for now. This commit should
pass for the following:

- mips32
- mips64
- musl32
- musl64
- android32
- android64
- arm32
- aarch64
- x86
- x86_64

Then we can figure out the powerpc variants.  This also prevents
potential errors for sparc64 which is not covered by CI.

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.

musl has a different aiocb definition; netbsd and OSX have some symbol renaming, and
lwpid_t is signed on freebsd