[libomptarget] Drop assert.h, use freestanding for amdgcn devicertl

[libomptarget] Drop assert.h, use freestanding for amdgcn devicertl

Promotes the runtime assert to a link time error for the unimplemented
fallback functions. Enables amdgcn to build with only clang provided
headers, which makes it less likely to break other builds when enabled.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D98649