AMDGPU: mark {exp,log}10{,f,l} library functions as unavailable

Summary:
The SimplifyLibCalls part of InstCombine generates calls to those otherwise.

I wonder if at some point we shouldn't just call disableAllFunctions() and
then enable functions on a whitelist basis...

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96495

Reviewers: arsenm, tstellarAMD

Subscribers: llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D21282

llvm-svn: 272664