The N32 ABI uses RELA relocation format, do not use 3-in-1 relocation's encoding, and uses ELFCLASS32. This change passes the IsN32 flag to the MCAsmBackend to distinguish usage of N32 ABI.
We still do not handle some cases like providing the -target-abi=o32 command line option with the mips64 target triple. That's why elf_header.s contains some "FIXME" strings. This case will be fixed in a separate patch.