cmd/link: add .go.buildinfo in XCOFF symbol table

.go.buildinfo must be added to the symbol table on AIX. Otherwise, ld
won't be able to handle its relocations.

This patch also make ".data" the default section for all symbols inside
the data segment.

Change-Id: I83ac2bf1050e0ef6ef9c96ff793efd4ddc8e98d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/174298
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -2265,11 +2265,23 @@ func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int6
 		}
 	}
 
-	for _, s := range ctxt.Syms.Allsym {
+	shouldBeInSymbolTable := func(s *sym.Symbol) bool {
 		if s.Attr.NotInSymbolTable() {
-			continue
+			return false
+		}
+		if ctxt.HeadType == objabi.Haix && s.Name == ".go.buildinfo" {
+			// On AIX, .go.buildinfo must be in the symbol table as
+			// it has relocations.
+			return true
 		}
 		if (s.Name == "" || s.Name[0] == '.') && !s.IsFileLocal() && s.Name != ".rathole" && s.Name != ".TOC." {
+			return false
+		}
+		return true
+	}
+
+	for _, s := range ctxt.Syms.Allsym {
+		if !shouldBeInSymbolTable(s) {
 			continue
 		}
 		switch s.Type {

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

@@ -513,16 +513,13 @@ func (f *xcoffFile) getXCOFFscnum(sect *sym.Section) int16 {
 	case &Segtext:
 		return f.sectNameToScnum[".text"]
 	case &Segdata:
-		if sect.Name == ".noptrdata" || sect.Name == ".data" {
-			return f.sectNameToScnum[".data"]
-		}
 		if sect.Name == ".noptrbss" || sect.Name == ".bss" {
 			return f.sectNameToScnum[".bss"]
 		}
 		if sect.Name == ".tbss" {
 			return f.sectNameToScnum[".tbss"]
 		}
-		Errorf(nil, "unknown XCOFF segment data section: %s", sect.Name)
+		return f.sectNameToScnum[".data"]
 	case &Segdwarf:
 		name, _ := xcoffGetDwarfSubtype(sect.Name)
 		return f.sectNameToScnum[name]