[mips] Fix MIPS N32 ABI triples support

Add support mips64(el)-linux-gnuabin32 triples, and set them to N32.
Debian architecture name mipsn32/mipsn32el are also added. Set
UseIntegratedAssembler for N32 if we can detect it.

Patch by YunQiang Su.

Differential revision: https://reviews.llvm.org/D51408

llvm-svn: 342416