[ELF][MIPS] Allow .MIPS.abiflags larger than one Elf_Mips_ABIFlags struct

Older versions of BFD generate libraries with .MIPS.abiflags that only
concatenate the individual .MIPS.abiflags sections instead of merging.

Patch by Alexander Richardson.

Differential revision: https://reviews.llvm.org/D27770

llvm-svn: 290237