[OpenMP][DeviceRTL][AMDGPU] Support code object version 5

Update DeviceRTL and the AMDGPU plugin to support code
object version 5. Default is code object version 4.

CodeGen for __builtin_amdgpu_workgroup_size generates code
for cov4 as well as cov5 if -mcode-object-version=none
is specified. DeviceRTL compilation passes this argument
via Xclang option to generate abi-agnostic code.

Generated code for the above builtin uses a clang
control constant "llvm.amdgcn.abi.version" to branch on
the abi version, which is available during linking of
user's OpenMP code. Load of this constant gets eliminated
during linking.

AMDGPU plugin queries the ELF for code object version
and then prepares various implicitargs accordingly.

Differential Revision: https://reviews.llvm.org/D139730

Reviewed By: jhuber6, yaxunl