gccrs: intrinsics: Implement atomic_load intrinsics

gcc/rust/ChangeLog:

	* backend/rust-builtins.cc (BuiltinsContext::setup_atomic_fns): Declare atomic
	load intrinsics.
	* backend/rust-compile-intrinsic.cc (atomic_load_handler_inner): New handler.
	(atomic_load_handler): Likewise.
	(unchecked_op_handler): Remove `static` function qualifier.
	(build_atomic_builtin_name): Handle load intrinsics.
	(atomic_store_handler_inner): New handler.

gcc/testsuite/ChangeLog:

	* rust/compile/torture/intrinsics-3.rs:
	* rust/execute/torture/atomic_load.rs: New test.