[mips] Use RegisterMCAsmBackend to register all MIPS asm backends. NFC

This change converts the `MipsAsmBackend` constructor to the "standard"
form. It makes possible to use `RegisterMCAsmBackend` for the backends
registrations. Now we pass `Triple` instance to the `MipsAsmBackend`
ctor and deduce all required options like endianness and bitness from
the triple. We still need to implement explicit ABI checking for
providing correct options to backends.

Differential revision: https://reviews.llvm.org/D37519

llvm-svn: 312720