AMDGPU: mark ldexp LibCalls as unavailable

Summary:
The LibCallSimplifier will turn llvm.exp2.* intrinsics into ldexp* libcalls
which do not make sense with the AMDGPU backend.

In the long run, we'll want an llvm.ldexp.* intrinsic to properly make use of
this optimization, but this works around the problem for now.

See also: http://reviews.llvm.org/D14327 (suggested llvm.ldexp.* implementation)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92709

Reviewers: arsenm, tstellarAMD

Differential Revision: http://reviews.llvm.org/D14990

llvm-svn: 255658