math: Add ilogb ported from amd-builtins

The scalar float/double function bodies are a direct copy/paste
with usage of the CLC wrappers to vectorize them.

This commit also adds in the FP_ILOGB0 and FP_ILOGBNAN macros which are
equal to the results of ilogb(0.0f) and ilogb(float nan) respectively.

v2: Add FP_ILOGB0 and FP_ILOGBNAN definitions

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
v1 Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

llvm-svn: 261639