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.

This and r293279 in LLVM resolves PR/23485.

Thanks to Brooks Davis for reporting the issue!

Reviewers: slthakur, seanbruno

Differential Revision: https://reviews.llvm.org/D29031

llvm-svn: 293285