test/codegen: enable more tests for ppc64/ppc64le

Adding cases for ppc64,ppc64le to the codegen tests
where appropriate.

Change-Id: Idf8cbe88a4ab4406a4ef1ea777bd15a58b68f3ed
Reviewed-on: https://go-review.googlesource.com/c/142557
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/codegen/comparisons.go

@@ -36,6 +36,7 @@ func CompareString2(s string) bool {
 func CompareString3(s string) bool {
 	// amd64:`CMPQ\t\(.*\), [A-Z]`
 	// arm64:-`CMPW\t`
+	// ppc64:-`CMPW\t`
 	// ppc64le:-`CMPW\t`
 	// s390x:-`CMPW\t`
 	return s == "xxxxxxxx"