cmd/compile: enable SSA for PowerPC 64 Big-endian

It passed tests once, if anything's wrong, better to fail
sooner than later.

Change-Id: Ibb1c5db3f4c5535a4ff4681fd157db77082c5041
Reviewed-on: https://go-review.googlesource.com/28982
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -40,7 +40,7 @@ func shouldssa(fn *Node) bool {
 		if os.Getenv("SSATEST") == "" {
 			return false
 		}
-	case "amd64", "amd64p32", "arm", "386", "arm64", "ppc64le", "mips64", "mips64le", "s390x":
+	case "amd64", "amd64p32", "arm", "386", "arm64", "ppc64", "ppc64le", "mips64", "mips64le", "s390x":
 		// Generally available.
 	}
 	if !ssaEnabled {

src/cmd/compile/internal/ssa/config.go

@@ -183,7 +183,7 @@ func NewConfig(arch string, fe Frontend, ctxt *obj.Link, optimize bool) *Config
 		c.FPReg = framepointerRegARM64
 		c.hasGReg = true
 		c.noDuffDevice = obj.GOOS == "darwin" // darwin linker cannot handle BR26 reloc with non-zero addend
-	case "ppc64le":
+	case "ppc64le", "ppc64":
 		c.IntSize = 8
 		c.PtrSize = 8
 		c.lowerBlock = rewriteBlockPPC64