Do not copy long double and 128-bit fp format from aux target for AMDGPU

rC352620 caused regressions because it copied floating point format from
aux target.

floating point format decides whether extended long double is supported.
It is x86_fp80 on x86 but IEEE double on amdgcn.

Document usage of long doubel type in HIP programming guide 
https://github.com/ROCm-Developer-Tools/HIP/pull/890

Differential Revision: https://reviews.llvm.org/D57527

llvm-svn: 352801