Use `llvm.ppc.altivec.lvx` intrinsic for `vec_ld`
Use `llvm.ppc.altivec.lvx` intrinsic for `vec_ld`

The `vec_ld` intrinsic is expected to automatically round the given
address down to a 16-byte boundary.

rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch