cmd/go,cmd/link: support buildmode c-shared on ppc64le

This change enables buildmode c-shared on ppc64le.

A bug was fixed in runtime/rt0_linux_ppc64le.s that was necessary to
make this work.  In _rt0_ppc64le_linux_lib, there is code to store
the value of r2 onto the caller's stack.  However, if this file
is compiled using a build mode that maintains the TOC address in
r2, then instructions will be inserted at the beginning of this
function to generate the r2 value for the callee, not the caller.
That means the r2 value for the callee is stored onto the caller's
stack.  If caller and callee don't have the same r2 values, then
the caller will restore the wrong r2 value after it returns.  This
situation can happen when using dlopen since the caller of this
function will be in ld64.so and will definitely have a different
TOC.

Updates #20756

Change-Id: I6e165e0d0716e73721bbbcc520e8302e4856e3ba
Reviewed-on: https://go-review.googlesource.com/53890Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -824,7 +824,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		return false
 	case "c-shared":
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64",
+		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le",
 			"darwin-amd64", "darwin-386",
 			"android-arm", "android-arm64", "android-386":
 			return true

src/cmd/go/internal/work/build.go

@@ -293,7 +293,7 @@ func BuildModeInit() {
 			codegenArg = "-fPIC"
 		} else {
 			switch platform {
-			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
+			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/ppc64le",
 				"android/amd64", "android/arm", "android/arm64", "android/386":
 				codegenArg = "-shared"
 			case "darwin/amd64", "darwin/386":

src/cmd/link/internal/ld/config.go

@@ -63,7 +63,7 @@ func (mode *BuildMode) Set(s string) error {
 		*mode = BuildmodeCArchive
 	case "c-shared":
 		switch objabi.GOARCH {
-		case "386", "amd64", "arm", "arm64":
+		case "386", "amd64", "arm", "arm64", "ppc64le":
 		default:
 			return badmode()
 		}

src/runtime/rt0_linux_ppc64le.s

@@ -10,7 +10,6 @@ TEXT _rt0_ppc64le_linux_lib(SB),NOSPLIT,$-8
 	MOVD	R0, 16(R1) // Save LR in caller's frame.
 	MOVW	CR, R0     // Save CR in caller's frame
 	MOVD	R0, 8(R1)
-	MOVD	R2, 24(R1) // Save TOC in caller's frame.
 	MOVDU	R1, -320(R1) // Allocate frame.
 	
 	// Preserve callee-save registers.
@@ -121,7 +120,6 @@ done:
 	FMOVD	304(R1), F31
 
 	ADD	$320, R1
-	MOVD	24(R1), R2
 	MOVD	8(R1), R0
 	MOVFL	R0, $0xff
 	MOVD	16(R1), R0