openmp: Do USM transform for omp_target_alloc

openmp: Do USM transform for omp_target_alloc

OpenMP 5.0 says that omp_target_alloc should return USM addresses.

gcc/ChangeLog:

	* omp-low.cc (usm_transform): Transform omp_target_alloc and
	omp_target_free.

libgomp/ChangeLog:

	* testsuite/libgomp.c/usm-6.c: Add omp_target_alloc.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/usm-2.c: Add omp_target_alloc.
	* c-c++-common/gomp/usm-3.c: Add omp_target_alloc.