Implement vec256/512

[TheNumbat]

Adds support for 256 and 512 bit vectors throughout the compiler. Generally has exactly the same semantics as 128 bit vectors, but larger. About half of the diff (+5k loc) is from enabling AVX/AVX2 in simdgen.

• 256-bit types are allowed with simd_beta. The basic tests for 128-bit have been duplicated for 256, but none of the operations in the other tests are implemented yet. AVX and AVX2 architecture extensions have been added and are enabled by default, since we already enable other Haswell extensions.
• 512-bit types are allowed with simd_alpha. They do not yet have tests and their usage will trigger a fatal error in emit. The AVX512F extension is also disabled by default.

I think the only changes that need careful review are those related to preserving/aligning wide registers (mainly amd64.S/emit):

• caml_call_gc and related functions now have versions that save ymm/zmm registers. Emit chooses which one to call based on the live register set. Programs that do not use 256/512 bit vectors should see identical behavior.
• When a C call includes vec256 or vec512 stack arguments, C compilers assume the stack will be 32/64 byte aligned upon entering the external call. Therefore we emit a call to caml_c_call_stack_args_avx{512}, which aligns the C stack before copying the arguments from the OCaml stack. We also do this in runtime4.

For later PRs:

• constants (+tests)
• static & reinterpret casts (+tests)
• array accessors (+tests)
• intrinsics (+tests)
• align vec256/512 stack slots on the ocaml stack
• use vex encoding for sse intrinsics when avx is enabled (evex if avx512)
• avx512 mask registers
• fix asan checks for 32/64 byte ops (and unaligned 16 byte)

[TheNumbat]

Review:

• @ccasin - typing, toplevel, testsuite, parsing, lambda, .github, flambda-backend
• @mshinwell - middle_end + cmm
• @xclerc - backend, tools, file_formats - cmm
• @NickBarnes - runtime, runtime4

[ccasin]

I've reviewed the directories assigned to me and they look fine, modulo my comments above - mainly I think we need to document clearly somewhere all the bits that are missing before we can consider moving this out of beta.

[mshinwell]

The middle_end/ changes are ok, I pushed a few small fixes.

[mshinwell]

New review assignment: I will read the Cmm parts, @xclerc will read the remainder of the backend.

[mshinwell]

I've now read backend/*cmm*

[TheNumbat]

Responded to review - now need to fix the asan job & some failure in stack checks + effects

[TheNumbat]

The stack checks failure is due to #4078; also added a wrong asan case to fix in a following pr.

[NickBarnes]

I've only reviewed the runtime side of this. It's broadly OK; various minor suggestions. In addition to these, I found some of the macrology in amd64.S hard to follow, so have made a suggested improvement in a commit here: https://github.com/NickBarnes/oxcaml/tree/avx-macrology

[NickBarnes]

The runtime stuff looks good to me now.