Lower FNEG ( FABS (x) ) -> FNABS (x) [X86 codegen] PR20578

Negative FABS of either a scalar or vector should be handled the same way
on x86 with SSE/AVX: a single OR instruction of the FP operand with a
constant to light up the sign bit(s).

http://llvm.org/bugs/show_bug.cgi?id=20578

Differential Revision: http://reviews.llvm.org/D5201
Rename bit_RDRAND to bit_RDRND to match GCC's version of this header.
Add the various signature macros.