openmp: Use libgomp memory allocation functions with unified shared memory.

openmp: Use libgomp memory allocation functions with unified shared memory.

This patches changes calls to malloc/free/calloc/realloc/aligned_alloc and
operator new to memory allocation functions in libgomp with
allocator=ompx_unified_shared_mem_alloc.  This helps existing code to benefit
from the unified shared memory.  The libgomp does the correct thing with all
the mapping constructs and there is no memory copies if the pointer is pointing
to unified shared memory.

We only replace replacable new operator and not the class member or placement new.

Backport of a patch posted at
https://gcc.gnu.org/pipermail/gcc-patches/2022-March/591353.html

gcc/ChangeLog:

	* omp-low.c (usm_transform): New function.
	(make_pass_usm_transform): Likewise.
	(class pass_usm_transform): New.
	* passes.def: Add pass_usm_transform.
	* tree-pass.h (make_pass_usm_transform): New declaration.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/usm-2.c: New test.
	* c-c++-common/gomp/usm-3.c: New test.
	* g++.dg/gomp/usm-1.C: New test.
	* g++.dg/gomp/usm-2.C: New test.
	* g++.dg/gomp/usm-3.C: New test.
	* gfortran.dg/gomp/usm-2.f90: New test.
	* gfortran.dg/gomp/usm-3.f90: New test.

libgomp/ChangeLog:

	* testsuite/libgomp.c/usm-6.c: New test.
	* testsuite/libgomp.c++/usm-1.C: Likewise.