[Clang] Adjust PIC handling for the AMDGPU ToolChain

The AMDGPU target only emits shared libraries currently. This patch
changes the handling of the PIC level to be managed in the
AMDGPUToolChain rather than having a special case for it. This causes
`--target=amdgcn--` to no longer set the PIC. This should be an
acceptable change since that doesn't use a correct toolchain anyway.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D142999