runtime: speed up eqstring

runtime: speed up eqstring

eqstring does not need to check the length of the strings.

6g

benchmark                              old ns/op     new ns/op     delta
BenchmarkCompareStringEqual            7.03          6.14          -12.66%
BenchmarkCompareStringIdentical        3.36          3.04          -9.52%

5g

benchmark                                 old ns/op     new ns/op     delta
BenchmarkCompareStringEqual               238           232           -2.52%
BenchmarkCompareStringIdentical           90.8          80.7          -11.12%

The equivalent PPC changes are in a separate commit
because I don't have the hardware to test them.

Change-Id: I292874324b9bbd9d24f57a390cfff8b550cdd53c
Reviewed-on: https://go-review.googlesource.com/3955
Reviewed-by: Keith Randall <khr@golang.org>