[mips] Correct the invocation of GAS in several cases.

Summary:
As a result of this patch, assembling an empty file with GCC and Clang (using
GAS as the assembler) now produces identical objects.

-mfp32/-mfpxx/-mfp64 now form a trinity of options. -mfpxx is the default
when the triple vendor is 'img' or 'mti', the ABI is O32, and the CPU is
between mips2 and mips32r2/mips64r2 (inclusive).

-mno-shared is always given to the assembler to match the effect of
-mabicalls (currently unimplemented but Clang acts as if it is given).
Similarly, -call_nonpic is always given to match the effect of -mplt (also
unimplemented and acts as if given) except when the ABI is 64 in which case
-mplt has no effect so -KPIC is given instead.

-mhard-float/-msoft-float are now passed on.

-modd-spreg/-mno-odd-spreg are now passed on.

-mno-mips16 is correctly passed on. The assembler option is -no-mips16 not
-mno-mips16

Differential Revision: http://reviews.llvm.org/D4515