[libomptarget][nfc] Add hook to easily disable building amdgcn bclib

[libomptarget][nfc] Add hook to easily disable building amdgcn bclib

This is useful when building LLVM with a toolchain that can't emit code
for amdgcn, e.g. because it overrides the include search path with headers
from another architecture, or the clang compiler is missing builtins.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D102229