Fix x86-64 __memset_chk.

I can only assume I was testing the 32-bit implementation when I claimed
this worked. While improving the 32-bit code I realized that I'd used
signed comparisons instead of unsigned, and came back to find that the
64-bit code didn't work.

By way of apology, make x86-64 the first architecture where __memset_chk
falls through to memset.

Change-Id: I54d9eee5349b6a2abb2ce81e161fdcde09556561