openmp: Implement OpenMP 5.1 atomics, so far for C only

This patch implements OpenMP 5.1 atomics (with clarifications from upcoming 5.2).
The most important changes are that it is now possible to write (for C/C++,
for Fortran it was possible before already) min/max atomics and more importantly
compare and exchange in various forms.
Also, acq_rel is now allowed on read/write and acq_rel/acquire are allowed on
update, and there are new compare, weak and fail clauses.

2021-09-10  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* tree-core.h (enum omp_memory_order): Add OMP_MEMORY_ORDER_MASK,
	OMP_FAIL_MEMORY_ORDER_UNSPECIFIED, OMP_FAIL_MEMORY_ORDER_RELAXED,
	OMP_FAIL_MEMORY_ORDER_ACQUIRE, OMP_FAIL_MEMORY_ORDER_RELEASE,
	OMP_FAIL_MEMORY_ORDER_ACQ_REL, OMP_FAIL_MEMORY_ORDER_SEQ_CST and
	OMP_FAIL_MEMORY_ORDER_MASK enumerators.
	(OMP_FAIL_MEMORY_ORDER_SHIFT): Define.
	* gimple-pretty-print.c (dump_gimple_omp_atomic_load,
	dump_gimple_omp_atomic_store): Print [weak] for weak atomic
	load/store.
	* gimple.h (enum gf_mask): Change GF_OMP_ATOMIC_MEMORY_ORDER
	to 6-bit mask, adjust GF_OMP_ATOMIC_NEED_VALUE value and add
	GF_OMP_ATOMIC_WEAK.
	(gimple_omp_atomic_weak_p, gimple_omp_atomic_set_weak): New inline
	functions.
	* tree.h (OMP_ATOMIC_WEAK): Define.
	* tree-pretty-print.c (dump_omp_atomic_memory_order): Adjust for
	fail memory order being encoded in the same enum and also print
	fail clause if present.
	(dump_generic_node): Print weak clause if OMP_ATOMIC_WEAK.
	* gimplify.c (goa_stabilize_expr): Add target_expr and rhs arguments,
	handle pre_p == NULL case as a test mode that only returns value
	but doesn't change gimplify nor change anything otherwise, adjust
	recursive calls, add MODIFY_EXPR, ADDR_EXPR, COND_EXPR, TARGET_EXPR
	and CALL_EXPR handling, adjust COMPOUND_EXPR handling for
	__builtin_clear_padding calls, for !rhs gimplify as lvalue rather
	than rvalue.
	(gimplify_omp_atomic): Adjust goa_stabilize_expr caller.  Handle
	COND_EXPR rhs.  Set weak flag on gimple load/store for
	OMP_ATOMIC_WEAK.
	* omp-expand.c (omp_memory_order_to_fail_memmodel): New function.
	(omp_memory_order_to_memmodel): Adjust for fail clause encoded
	in the same enum.
	(expand_omp_atomic_cas): New function.
	(expand_omp_atomic_pipeline): Use omp_memory_order_to_fail_memmodel
	function.
	(expand_omp_atomic): Attempt to optimize atomic compare and exchange
	using expand_omp_atomic_cas.
gcc/c-family/
	* c-common.h (c_finish_omp_atomic): Add r and weak arguments.
	* c-omp.c: Include gimple-fold.h.
	(c_finish_omp_atomic): Add r and weak arguments.  Add support for
	OpenMP 5.1 atomics.
gcc/c/
	* c-parser.c (c_parser_conditional_expression): If omp_atomic_lhs and
	cond.value is >, < or == with omp_atomic_lhs as one of the operands,
	don't call build_conditional_expr, instead build a COND_EXPR directly.
	(c_parser_binary_expression): Avoid calling parser_build_binary_op
	if omp_atomic_lhs even in more cases for >, < or ==.
	(c_parser_omp_atomic): Update function comment for OpenMP 5.1 atomics,
	parse OpenMP 5.1 atomics and fail, compare and weak clauses, allow
	acq_rel on atomic read/write and acq_rel/acquire clauses on update.
	* c-typeck.c (build_binary_op): For flag_openmp only handle
	MIN_EXPR/MAX_EXPR.
gcc/cp/
	* parser.c (cp_parser_omp_atomic): Allow acq_rel on atomic read/write
	and acq_rel/acquire clauses on update.
	* semantics.c (finish_omp_atomic): Adjust c_finish_omp_atomic caller.
gcc/testsuite/
	* c-c++-common/gomp/atomic-17.c (foo): Add tests for atomic read,
	write or update with acq_rel clause and atomic update with acquire clause.
	* c-c++-common/gomp/atomic-18.c (foo): Adjust expected diagnostics
	wording, remove tests moved to atomic-17.c.
	* c-c++-common/gomp/atomic-21.c: Expect only 2 omp atomic release and
	2 omp atomic acq_rel directives instead of 4 omp atomic release.
	* c-c++-common/gomp/atomic-25.c: New test.
	* c-c++-common/gomp/atomic-26.c: New test.
	* c-c++-common/gomp/atomic-27.c: New test.
	* c-c++-common/gomp/atomic-28.c: New test.
	* c-c++-common/gomp/atomic-29.c: New test.
	* c-c++-common/gomp/atomic-30.c: New test.
	* c-c++-common/goacc-gomp/atomic.c: Expect 1 omp atomic release and
	1 omp atomic_acq_rel instead of 2 omp atomic release directives.
	* gcc.dg/gomp/atomic-5.c: Adjust expected error diagnostic wording.
	* g++.dg/gomp/atomic-18.C:Expect 4 omp atomic release and
	1 omp atomic_acq_rel instead of 5 omp atomic release directives.
libgomp/
	* testsuite/libgomp.c-c++-common/atomic-19.c: New test.
	* testsuite/libgomp.c-c++-common/atomic-20.c: New test.
	* testsuite/libgomp.c-c++-common/atomic-21.c: New test.