Introduce atomic loads in Cmm and Mach IRs

[OlivierNicole]

The Patomic_load primitive gives lieu to a non-atomic load (i.e. Cload { is_atomic = false; _ }) in Cmm. This is correct in x86 but needs to be lowered down the pipeline to ease support of other architectures.

In addition, this PR proposes to add an ~is_atomic argument to the mk_load_mut helper to make the atomicity of loads explicit during Cmm generation.

[kayceesrk]

Note that even with this change, the Cmm to Mach translation ignores the atomicity.

ocaml/asmcomp/selectgen.ml

Lines 449 to 451 in 488a28a

	| (Cload {memory_chunk; mutability}, [arg]) ->
	let (addr, eloc) = self#select_addressing memory_chunk arg in
	(Iload(memory_chunk, addr, mutability), [eloc])

Further work is needed to incorporate the atomicity argument into the definition of Iload.

ocaml/asmcomp/mach.mli

Line 55 in 488a28a

| Iload of Cmm.memory_chunk * Arch.addressing_mode * Asttypes.mutable_flag

It is useful to incorporate this change into this PR. Best to change Iload into an inline record for readability. Perhaps @gasche may have other thoughts before I ask you to do more work.

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On a related note, the atomic store, fetch-and-add and compare-and-swap are also converted early into an external calls in cmmgen. This may also need to be lowered for efficient codegen.

ocaml/asmcomp/cmmgen.ml

Lines 1056 to 1061 in 488a28a

	| Patomic_exchange ->
	Cop (Cextcall ("caml_atomic_exchange", typ_val, [], false),
	[transl env arg1; transl env arg2], dbg)
	| Patomic_fetch_add ->
	Cop (Cextcall ("caml_atomic_fetch_add", typ_int, [], false),
	[transl env arg1; transl env arg2], dbg)

ocaml/asmcomp/cmmgen.ml

Lines 1115 to 1117 in 488a28a

	| Patomic_cas ->
	Cop (Cextcall ("caml_atomic_cas", typ_int, [], false),
	[transl env arg1; transl env arg2; transl env arg3], dbg)

[OlivierNicole]

It is useful to incorporate this change into this PR. Best to change Iload into an inline record for readability.

This is the case in the new version.

[kayceesrk]

The changes look good to me.

[kayceesrk]

LGTM.

I've left a few comments that clarify the API.

[OlivierNicole]

I integrated your comments.

[gasche]

Thanks! This looks good, so I merged.