[release-branch.go1.8] cmd/link: put plt stubs first in Textp on ppc64x

Previously call stubs were generated and inserted in
Textp after much of the text, resulting in calls too
far in some cases. This puts the call stubs first, which
in many cases makes some calls not so far, but also
enables trampolines to be generated when necessary.

This is a backport for go 1.8 based on CL38131.

Fixes #19578

Change-Id: If3ba3d5222a7f7969ed2de1df4854a1b4a80a0f0
Reviewed-on: https://go-review.googlesource.com/38472
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/ppc64/asm.go

@@ -87,6 +87,7 @@ func genplt(ctxt *ld.Link) {
 	//
 	// This assumes "case 1" from the ABI, where the caller needs
 	// us to save and restore the TOC pointer.
+	var stubs []*ld.Symbol
 	for _, s := range ctxt.Textp {
 		for i := range s.R {
 			r := &s.R[i]
@@ -108,7 +109,7 @@ func genplt(ctxt *ld.Link) {
 			if stub.Size == 0 {
 				// Need outer to resolve .TOC.
 				stub.Outer = s
-				ctxt.Textp = append(ctxt.Textp, stub)
+				stubs = append(stubs, stub)
 				gencallstub(ctxt, 1, stub, r.Sym)
 			}
 
@@ -121,6 +122,11 @@ func genplt(ctxt *ld.Link) {
 			ctxt.Arch.ByteOrder.PutUint32(s.P[r.Off+4:], o1)
 		}
 	}
+	// Put call stubs at the beginning (instead of the end).
+	// So when resolving the relocations to calls to the stubs,
+	// the addresses are known and trampolines can be inserted
+	// when necessary.
+	ctxt.Textp = append(stubs, ctxt.Textp...)
 }
 
 func genaddmoduledata(ctxt *ld.Link) {