amdgcn, libgomp: custom USM allocator

amdgcn, libgomp: custom USM allocator

There were problems with critical driver data sharing pages with USM data, so
this new allocator implementation moves USM to entirely different pages.

libgomp/ChangeLog:

	* plugin/plugin-gcn.c: Include sys/mman.h and unistd.h.
	(usm_heap_create): New function.
	(struct usm_splay_tree_key_s): Delete function.
	(usm_splay_compare): Delete function.
	(splay_tree_prefix): Delete define.
	(GOMP_OFFLOAD_usm_alloc): Use new allocator.
	(GOMP_OFFLOAD_usm_free): Likewise.
	(GOMP_OFFLOAD_is_usm_ptr): Likewise.
	(gomp_fatal): Delete macro.
	(splay_tree_c): Delete.
	* usm-allocator.c: New file.