[ELF][MIPS] Take into account combination of EF_MIPS_ARCH and EF_MIPS_MACH flags while checking ISA compatibility

MIPS ISA encoded using two ELF flags: general architecture flag like
EF_MIPS_ARCH_32, EF_MIPS_ARCH_64R6 etc and optional machine variant flag
like EF_MIPS_MACH_4111, EF_MIPS_MACH_OCTEON3 etc. When we check
compatibility between two input files and deduce ELF flags for generated
output we need to take into account both of these flags.

llvm-svn: 278230