Uncomment AVX512 byte vector conversions

Resolves my comment in rust-lang#197, at least for now; rust-lang#187 is pending but since these are already here, just commented, it seemed to make sense to me to re-enable them anyway.

crates/core_simd/src/vendor/x86.rs

@@ -8,10 +8,10 @@ use core::arch::x86_64::*;
 
 from_transmute! { unsafe u8x16 => __m128i }
 from_transmute! { unsafe u8x32 => __m256i }
-//from_transmute! { unsafe u8x64 => __m512i }
+from_transmute! { unsafe u8x64 => __m512i }
 from_transmute! { unsafe i8x16 => __m128i }
 from_transmute! { unsafe i8x32 => __m256i }
-//from_transmute! { unsafe i8x64 => __m512i }
+from_transmute! { unsafe i8x64 => __m512i }
 
 from_transmute! { unsafe u16x8 => __m128i }
 from_transmute! { unsafe u16x16 => __m256i }