cmd/link: implement -buildid for non-ELF binaries

Non-ELF binary formats are much less flexible and typically do not
have a good place to store the build ID.

We store it as raw bytes at the beginning of the text segment.

The only system I know of that will be upset about this is NaCl,
and NaCl is an ELF system and does not use this.

For #11048.

Change-Id: Iaa7ace703c4cf36392e752eea9b55e2ce49e9826
Reviewed-on: https://go-review.googlesource.com/10708Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -37,6 +37,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"strconv"
 	"strings"
 )
 
@@ -1546,6 +1547,29 @@ func dodata() {
 	}
 }
 
+// Add buildid to beginning of text segment, on non-ELF systems.
+// Non-ELF binary formats are not always flexible enough to
+// give us a place to put the Go build ID. On those systems, we put it
+// at the very beginning of the text segment.
+// This ``header'' is read by cmd/go.
+func textbuildid() {
+	if Iself || buildid == "" {
+		return
+	}
+
+	sym := Linklookup(Ctxt, "go.buildid", 0)
+	sym.Reachable = true
+	// The \xff is invalid UTF-8, meant to make it less likely
+	// to find one of these accidentally.
+	data := "\xff Go build ID: " + strconv.Quote(buildid) + "\n \xff"
+	sym.Type = obj.STEXT
+	sym.P = []byte(data)
+	sym.Size = int64(len(sym.P))
+
+	sym.Next = Ctxt.Textp
+	Ctxt.Textp = sym
+}
+
 // assign addresses to text
 func textaddress() {
 	var sub *LSym

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

@@ -231,6 +231,7 @@ func Ldmain() {
 	}
 	addexport()
 	Thearch.Gentext() // trampolines, call stubs, etc.
+	textbuildid()
 	textaddress()
 	pclntab()
 	findfunctab()