amdgpu/math: Don't use llvm instrinsic for native_log

AMDGPU targets don't have insturction for it,
so it'll be expanded to C * log2 anyway.

v2: use native_log2 instead of the more precise sw implementation
v3: move to amdgpu
v4: drop old AMD copyright

Reviewer: Aaron Watry
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 316587