[X86] Synchronize the encodekey builtins with gcc. Don't assume void* is 16 byte aligned.

We were taking multiple pointer arguments in the builtin.
gcc accepts a single void*.

The cast from void* to _m128i* caused the IR generation to assume
the pointer was aligned.

Instead make the builtin take a single void*, emit i8* GEPs to
adjust then cast to <2 x i64>* and perform a store with align of 1.