openmp: Change to using a hashtab to lookup offload target addresses for indirect function calls

A splay-tree was previously used to lookup equivalent target addresses
for a given host address on offload targets. However, as splay-trees can
modify their structure on lookup, they are not suitable for concurrent
access from separate teams/threads without some form of locking.  This
patch changes the lookup data structure to a hashtab instead, which does
not have these issues.

The call to build_indirect_map to initialize the data structure is now
called from just the first thread of the first team to avoid redundant
calls to this function.

2024-03-22  Kwok Cheung Yeung  <kcyeung@baylibre.com>

	libgomp/
	* config/accel/target-indirect.c: Include string.h and hashtab.h.
	Remove include of splay-tree.h.  Update comments.
	(splay_tree_prefix, splay_tree_c): Delete.
	(struct indirect_map_t): New.
	(hash_entry_type, htab_alloc, htab_free, htab_hash, htab_eq): New.
	(GOMP_INDIRECT_ADD_MAP): Remove volatile qualifier.
	(USE_SPLAY_TREE_LOOKUP): Rename to...
	(USE_HASHTAB_LOOKUP): ..this.
	(indirect_map, indirect_array): Delete.
	(indirect_htab): New.
	(build_indirect_map): Remove locking.  Build indirect map using
	hashtab.
	(GOMP_target_map_indirect_ptr): Use indirect_htab to lookup target
	address.
	(GOMP_target_map_indirect_ptr): Remove volatile qualifier.
	* config/gcn/team.c (gomp_gcn_enter_kernel): Call build_indirect_map
	from first thread of first team only.
	* config/nvptx/team.c (gomp_nvptx_main): Likewise.
	* testsuite/libgomp.c-c++-common/declare-target-indirect-2.c (main):
	Add missing break statements.
	* testsuite/libgomp.fortran/declare-target-indirect-2.f90: Remove
	xfail.