Load a single element and splat to all lanes of a v128 vector.
While this intrinsic is provided for completeness it can also be replaced
u8x16_splat(*m) and it should generate equivalent code (and also not
This intrinsic is unsafe because it takes a raw pointer as an argument, and the pointer must be valid to load 1 byte from. Note that there is no alignment requirement on this pointer since this intrinsic performs a 1-aligned load.