[mips] Pass the ABI to the integrated assembler and add tests the existing arguments.

[mips] Pass the ABI to the integrated assembler and add tests the existing arguments.

Summary:
With this patch (and a corresponding LLVM patch), assembling an empty file with
GCC and Clang -fintegrated-as produce near identical objects. The remaining
differences are:
* GCC/GAS produce objects have a .pdr section
* GCC/GAS produce objects have a .gnu.attributes section
Other differences are insignificant such as precise file offsets and the order
of strings in the string table.

Differential Revision: http://reviews.llvm.org/D4531

llvm-svn: 213241