Avoid targeting a partial register with load;
ensure source of load (writebarrier) is aligned.

Better yet would be "CMPB $1,writebarrier" but that requires
wrestling with flagalloc (mem operand complicates moving
instruction around).

Didn't see a change in time for
   benchcmd -n 10 Build go build net/http

Verified that we clean the code up properly:
   0x20a8 <main.main+104>:	mov    0xc30a2(%rip),%eax
                            # 0xc5150 <runtime.writeBarrier>
   0x20ae <main.main+110>:	test   %al,%al

Change-Id: Id5fb8c260eaec27bd727cb0ae1476c60343b0986
Reviewed-on: https://go-review.googlesource.com/19998Reviewed-by: Keith Randall <khr@golang.org>