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.