Fix the stack-protector test for x86/x86-64.

Built for fugu this was working fine, but built for generic x86/x86-64
the compiler was (a) optimizing out all the stack writes and (b) inserting
enough padding on x86-64 for the canary to be safely out of the way.

While here, let's tighten up this test so we test that it's sufficient to
only overwrite the buffer by one byte...

(cherry-pick of 6f90c1ac.)

Bug: http://b/27815668
Change-Id: I80a646de4b30fd5c78df20fdaa7e3eb163585caf