[mips] Set pointer size to 4 bytes for N32 ABI

CodePointerSize and CalleeSaveStackSlotSize values are used in DWARF
generation. In case of MIPS it's incorrect to check for Triple::isMIPS64()
only this function returns true for N32 ABI too.

Now we do not have a method to recognize N32 if it's specified by a command
line option and is not a part of a target triple. So we check for
Triple::GNUABIN32 only. It's better than nothing.

Differential revision: https://reviews.llvm.org/D52874

llvm-svn: 344039