Remove repr(simd) attribute and use a lang-item instead

[gnzlbg]

After #63531, we should, at some point, expose a #[lang = "packed_simd"] struct Simd<T, const N: usize>([T; N]); from libcore that we handle specially in the compiler instead of using a #[repr(simd)] attribute for it.

[workingjubilee]

The relevant PR landed in #78863.
Current issues that deal with #[repr(simd)] and thus may impact or motivate the resolution of this issue:

• Compile constant SIMD initialiser to a constant vector expression  #18147
• What to do about repr(C, simd)? #47103
• repr(simd) is unsound in C FFI #53346
• Invalid monomorphization when -C link-dead-code is used #77529
• Broken LLVM intrinsics with SIMD FFI on riscv64gc #77866
• repr(simd) does not align to Intel recs on x86_64 #81931