go/ssa/interp: add two intrinsics to fix tests

(specifically: strings.Count, testing.callerEntry)

The interpreter tests were very useful for finding bugs during
development of go/ssa but now seem to be all cost and no benefit.
It may be time to delete this package.

Change-Id: I22348be9fb37bb0fd0c572c3e6f57e70fc069e02
Reviewed-on: https://go-review.googlesource.com/40871
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed