runtime: fix ppc64le c-archive init

We weren't setting r0 to 0, as required by our generated code.

Before this patch, the misc/cgo/testcarchive tests failed on ppc64le.
After this patch, they work, so enable them.

Change-Id: I53b16746961da9f7c34f59030a1e40953c9c1e05
Reviewed-on: https://go-review.googlesource.com/44093
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -772,7 +772,8 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		}
 		switch pair {
 		case "darwin-386", "darwin-amd64", "darwin-arm", "darwin-arm64",
-			"linux-amd64", "linux-386", "windows-amd64", "windows-386":
+			"linux-amd64", "linux-386", "linux-ppc64le",
+			"windows-amd64", "windows-386":
 			return true
 		}
 		return false

src/runtime/rt0_linux_ppc64le.s

@@ -53,6 +53,9 @@ TEXT _rt0_ppc64le_linux_lib(SB),NOSPLIT,$-8
 	MOVD	R4, _rt0_ppc64le_linux_lib_argv<>(SB)
 
 	// Synchronous initialization.
+	MOVD	$runtime·reginit(SB), R12
+	MOVD	R12, CTR
+	BL	(CTR)
 	MOVD	$runtime·libpreinit(SB), R12
 	MOVD	R12, CTR
 	BL	(CTR)