Fix compilation warnings

[eightbitraptor]

Currently the third-party-heap branch does not compile with clang because clang uses -Werror=shorten-64-to-32 whereas GCC does not.

This PR fixes compilation with clang, and addresses most warnings when using either GCC or clang, when compiled with MMTk enabled and not.

Note:
There is still one warning present when using the GCC 12 included in Fedora 36. I have not addressed this as part of this PR as it's present on the upstream master branch.

Outstanding warning is as follows:

compiling gc.c
gc.c: In function ‘objspace_xfree’:
gc.c:12278:33: warning: pointer ‘ptr’ used after ‘free’ [-Wuse-after-free]
12278 |          malloc_increase_done = objspace_malloc_increase_body(__VA_ARGS__))
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gc.c:12591:5: note: in expansion of macro ‘objspace_malloc_increase’
12591 |     objspace_malloc_increase(objspace, ptr, 0, old_size, MEMOP_TYPE_FREE) {
      |     ^~~~~~~~~~~~~~~~~~~~~~~~
gc.c:12592:9: note: call to ‘free’ here
12592 |         free(ptr);
      |         ^~~~~~~~~

[wks]

Clang did a good job finding a bug in mmtk.h. The bytes parameter of mmtk_post_alloc should have been size_t, because it is usize on the Rust side. The semantics parameter is an enum on the Rust side, but it has an explicit #[repr(i32)] annotation so it is compatible with int32_t in C, and int is usually 32-bit integer.

#[no_mangle]
pub extern "C" fn mmtk_post_alloc(
    mutator: *mut Mutator<Ruby>,
    refer: ObjectReference,
    bytes: usize,
    semantics: AllocationSemantics,
) {
    memory_manager::post_alloc::<Ruby>(unsafe { &mut *mutator }, refer, bytes, semantics)
}

[wks]

LGTM

[wks]

I merged the PR. There is still some error when the CI runs the MJIT tests, but other tests passed. We can look into the MJIT errors later.