[AMDGPU] add __builtin_amdgcn_s_getpc

Summary: Added the builtin corresponding to the s_getpc intrinsic added in llvm D32862

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye

Differential Revision: https://reviews.llvm.org/D33276

llvm-svn: 303861