cmd/link: correct byte ordering in plan9_arm object header

Fields in Plan 9 object headers are big-endian, on all architectures.

Change-Id: If95ad29750b776338178d660646568bf26a4abda
Reviewed-on: https://go-review.googlesource.com/18964Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -689,14 +689,14 @@ func asmb() {
 	switch ld.HEADTYPE {
 	default:
 	case obj.Hplan9: /* plan 9 */
-		ld.Thearch.Lput(0x647)                      /* magic */
-		ld.Thearch.Lput(uint32(ld.Segtext.Filelen)) /* sizes */
-		ld.Thearch.Lput(uint32(ld.Segdata.Filelen))
-		ld.Thearch.Lput(uint32(ld.Segdata.Length - ld.Segdata.Filelen))
-		ld.Thearch.Lput(uint32(ld.Symsize))      /* nsyms */
-		ld.Thearch.Lput(uint32(ld.Entryvalue())) /* va of entry */
-		ld.Thearch.Lput(0)
-		ld.Thearch.Lput(uint32(ld.Lcsize))
+		ld.Lputb(0x647)                      /* magic */
+		ld.Lputb(uint32(ld.Segtext.Filelen)) /* sizes */
+		ld.Lputb(uint32(ld.Segdata.Filelen))
+		ld.Lputb(uint32(ld.Segdata.Length - ld.Segdata.Filelen))
+		ld.Lputb(uint32(ld.Symsize))      /* nsyms */
+		ld.Lputb(uint32(ld.Entryvalue())) /* va of entry */
+		ld.Lputb(0)
+		ld.Lputb(uint32(ld.Lcsize))
 
 	case obj.Hlinux,
 		obj.Hfreebsd,

src/runtime/asm_arm.s

@@ -81,12 +81,17 @@ DATA	runtime·mainPC+0(SB)/4,$runtime·main(SB)
 GLOBL	runtime·mainPC(SB),RODATA,$4
 
 TEXT runtime·breakpoint(SB),NOSPLIT,$0-0
+	BL		runtime·emptyfunc(SB)	// force R14 save for traceback
 	// gdb won't skip this breakpoint instruction automatically,
 	// so you must manually "set $pc+=4" to skip it and continue.
 #ifdef GOOS_nacl
 	WORD	$0xe125be7f	// BKPT 0x5bef, NACL_INSTR_ARM_BREAKPOINT
+#else
+#ifdef GOOS_plan9
+	WORD	$0xD1200070	// undefined instruction used as armv5 breakpoint in Plan 9
 #else
 	WORD	$0xe7f001f0	// undefined instruction that gdb understands is a software breakpoint
+#endif
 #endif
 	RET