[TLI][AMDGPU] AMDPAL does not have library functions

Configure TLI to say that r600/amdgpu does not have any library
functions, such that InstCombine does not do anything like turn sin/cos
into the library function @tan with sufficient fast math flags.

Differential Revision: https://reviews.llvm.org/D67406

Change-Id: I02f907d3e64832117ea9800e9f9285282856e5df
llvm-svn: 371592