remove mips-only EINIT and EREMDEV errnos

the errno values are unused by the kernel and the macro definitions were
never exposed by glibc.

fix numerous mips abi constant definition mismatches

initial version of mips (o32) port, based on work by Richard Pennington (rdp)

basically, this version of the code was obtained by starting with
rdp's work from his ellcc source tree, adapting it to musl's build
system and coding style, auditing the bits headers for discrepencies
with kernel definitions or glibc/LSB ABI or large file issues, fixing
up incompatibility with the old binutils from aboriginal linux, and
adding some new special cases to deal with the oddities of sigaction
and pipe syscall interfaces on mips.

at present, minimal test programs work, but some interfaces are broken
or missing. threaded programs probably will not link.