eqstring does not need to check the length of the strings.
Other architectures were done in a separate commit.

While we're here, add a pointer equality check.

Change-Id: Id2c8616a03a7da7037c1e9ccd56a549fc952bd98
Reviewed-on: https://go-review.googlesource.com/3956Reviewed-by: Keith Randall <khr@golang.org>