math: Implement native_log10

Use llvm instrinsic by default
Provide amdgpu workaround

v2: drop old amd copyrights

Reviewer: Aaron Watry
Reviewed-by: Vedran Miletić <vedran@miletic.net>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 316588