The patch teaches the Clang driver how to handle the N64 static
relocation model properly. It enforces the correct target feature
(+noabicalls) when -fno-pic is used. This is required as non-pic
N64 code as the abi extension to call PIC code (CPIC) is unsupported.
Make PIC the default for mips64 and mips64el, this affects both N32
& N64 ABIs, to better match GCC.
As part of this effort, clean up the assembler invocation command
builder, so the correct flags are used.