[mips] Fix ELF e_flags related to -mabicalls and -mplt.

Summary:
These options are not implemented yet but we act as if they are always
given.

The integrated assembler is driven by the clang driver so the e_flag test
cases should match the e_flags emitted by GCC+GAS rather than GAS
by itself.

Differential Revision: http://reviews.llvm.org/D4536