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.

Runtime changes:

• 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_32/64, 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