OpenMP: Support accelerated 2D/3D memory copies for AMD GCN

This patch adds support for 2D/3D memory copies for omp_target_memcpy_rect
using AMD extensions to the HSA API.  This is just the AMD GCN-specific
part of the following patch:

  https://gcc.gnu.org/pipermail/gcc-patches/2023-September/631001.html

2024-01-04  Julian Brown  <julian@codesourcery.com>

libgomp/
	* plugin/plugin-gcn.c (hsa_runtime_fn_info): Add
	hsa_amd_memory_lock_fn, hsa_amd_memory_unlock_fn,
	hsa_amd_memory_async_copy_rect_fn function pointers.
	(init_hsa_runtime_functions): Add above functions, with
	DLSYM_OPT_FN.
	(GOMP_OFFLOAD_memcpy2d, GOMP_OFFLOAD_memcpy3d): New functions.