liblink, cmd/link: add version number to object file

There are changes we know we want to make, but not before Go 1.3
Add a version number so that we can make them more easily later.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/87670043

src/cmd/link/testdata/Makefile

-all: hello.6 pclntab.6
+ALL=\
+	autosection.6\
+	autoweak.6\
+	dead.6\
+	hello.6\
+	layout.6\
+	pclntab.6\
 
-hello.6: hello.s
-	go tool 6a hello.s
+all: $(ALL)
 
-pclntab.6: pclntab.s
-	go tool 6a pclntab.s
+%.6: %.s
+	go tool 6a $*.s
 
 pclntab.s: genpcln.go
 	go run genpcln.go >pclntab.s

src/cmd/link/testdata/link.hello.darwin.amd64

@@ -49,8 +49,8 @@
 00001080  02 20 00 04 20 00 06 05  02 05 02 05 02 05 02 02  |. .. ...........|
 00001090  02 02 02 05 02 02 02 01  00 00 00 00 00 00 00 00  |................|
 000010a0  02 00 00 00 88 00 00 00  2f 55 73 65 72 73 2f 72  |......../Users/r|
-000010b0  73 63 2f 72 73 63 67 6f  2f 73 72 63 2f 63 6d 64  |sc/rscgo/src/cmd|
-000010c0  2f 6c 64 32 2f 74 65 73  74 64 61 74 61 2f 68 65  |/ld2/testdata/he|
+000010b0  73 63 2f 67 2f 67 6f 2f  73 72 63 2f 63 6d 64 2f  |sc/g/go/src/cmd/|
+000010c0  6c 69 6e 6b 2f 74 65 73  74 64 61 74 61 2f 68 65  |link/testdata/he|
 000010d0  6c 6c 6f 2e 73 00 00 00  00 00 00 00 00 00 00 00  |llo.s...........|
 *
 00002000  68 65 6c 6c 6f 20 77 6f  72 6c 64 0a              |hello world.|

src/liblink/objfile.c

@@ -16,6 +16,7 @@
 // The file format is:
 //
 //	- magic header: "\x00\x00go13ld"
+//	- byte 1 - version number
 //	- sequence of strings giving dependencies (imported packages)
 //	- empty string (marks end of sequence)
 //	- sequence of defined symbols
@@ -248,6 +249,7 @@ linkwriteobj(Link *ctxt, Biobuf *b)
 	Bputc(b, 0);
 	Bputc(b, 0);
 	Bprint(b, "go13ld");
+	Bputc(b, 1); // version
 
 	// Emit autolib.
 	for(h = ctxt->hist; h != nil; h = h->link)
@@ -453,6 +455,8 @@ ldobjfile(Link *ctxt, Biobuf *f, char *pkg, int64 len, char *pn)
 	Bread(f, buf, sizeof buf);
 	if(memcmp(buf, startmagic, sizeof buf) != 0)
 		sysfatal("%s: invalid file start %x %x %x %x %x %x %x %x", pn, buf[0], buf[1], buf[2], buf[3], buf[4], buf[5], buf[6], buf[7]);
+	if((c = Bgetc(f)) != 1)
+		sysfatal("%s: invalid file version number %d", pn, c);
 
 	for(;;) {
 		lib = rdstring(f);

src/pkg/debug/goobj/read.go

@@ -573,6 +573,11 @@ func (r *objReader) parseObject(prefix []byte) error {
 		return r.error(errCorruptObject)
 	}
 
+	b := r.readByte()
+	if b != 1 {
+		return r.error(errCorruptObject)
+	}
+
 	// Direct package dependencies.
 	for {
 		s := r.readString()