Define unaligned vector types in stdarch

Some C compilers (clang and gcc at least) define unaligned vector types. Vectors are often read from unaligned locations, and then using and dereferencing a pointer to the normal vector types would be UB. Hence these types:

// emmintrin.h
typedef double __m128d_u __attribute__((__vector_size__(16), __aligned__(1)));
typedef long long __m128i_u __attribute__((__vector_size__(16), __aligned__(1)));

// xmmintrin.h
typedef float __m128_u __attribute__((__vector_size__(16), __aligned__(1)));

// avxintrin.h
typedef float __m256_u __attribute__ ((__vector_size__ (32), __aligned__(1)));
typedef double __m256d_u __attribute__((__vector_size__(32), __aligned__(1)));
typedef long long __m256i_u __attribute__((__vector_size__(32), __aligned__(1)));

// avx512intrin.h
typedef float __m512_u __attribute__((__vector_size__(64), __aligned__(1)));
typedef double __m512d_u __attribute__((__vector_size__(64), __aligned__(1)));
typedef long long __m512i_u __attribute__((__vector_size__(64), __aligned__(1)));

They have the size of the standard vector types, but an alignment of 1. These types are useful for translating C to rust. e.g. https://github.com/immunant/c2rust/pull/1132.

Proposal

Define these types as one of these

#[repr(C)]
struct([u8; 16]);

#[repr(C)]
struct __m128d_u(u8, u8, u8, ..., u8);

#[repr(C, packed(1))]
struct __m128d_u(f64, f64);

I think the first option runs into arrays technically not being FFI-safe. Option 2 is kind of cumbersome, so I'd like to explore option 3 using packed. The packed option can be a little weird, but I don't think that matters here. The whole purpose of this type is just to load the value, and then it'll likely just get transmuted to a proper vector type.

I can open this as an ACP if that is the proper route.

Start by comparing the unaligned vector definitions in emmintrin.h, xmmintrin.h, avxintrin.h, and avx512intrin.h with stdarch's existing vector types. Evaluate the proposed Rust representations, especially repr(C, packed(1)), against the required sizes and alignment. Done means the corresponding unaligned types are defined with correct layout and their FFI safety and intended transmutation use are resolved.