[mips] Correct .MIPS.abiflags for -mfpxx on MIPS32r6

Summary:
The cpr1_size field describes the minimum register width to run the program
rather than the size of the registers on the target. MIPS32r6 was acting
as if -mfp64 has been given because it starts off with 64-bit FPU registers.

Differential Revision: http://reviews.llvm.org/D4538