[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.
clang-tidy: warning: invalid case style for function '__kmpc_impl_shfl_sync' [readability-identifier-naming]
not useful