libclc/math: Add cospi

Ported from the libclc/amd-builtins branch

v2: Rename sincos_f_piby4 to __libclc__sincosf_piby4
    Add cospi(double) implementation instead of using llvm.cos

Notes:
The sincosD_piby4.h file is mostly the same as the builtin implementation
released by AMD. The inline attribute declaration is changed, and M_PI is
used instead of a constant double. Otherwise, the only difference is that
the header explicitly enables the fp64 pragma.

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jeroen Ketema <j.ketema@imperial.ac.uk>
CC: Tom Stellard <tom@stellard.net>
CC: Matt Arsenault <Matthew.Arsenault@amd.com>
llvm-svn: 230641