[compiler-rt][mips] Add support for quad precision builtins for mips64

Match the builtins that GCC provides for IEEE754 quad precision
on MIPS64. Also, enable building them with clang as PR20098 is resolved.

Disable tests for xf and xc modes as MIPS doesn't support that mode in
hardware or software.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D32794

llvm-svn: 302147