cmd/link/internal/ld: add support for GOARCH=mips{,le}

Change-Id: Ida214ccc5858969ea60abb0787f4d98bab4336d6
Reviewed-on: https://go-review.googlesource.com/31480
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -946,7 +946,8 @@ func Elfinit(ctxt *Link) {
 		ehdr.shentsize = ELF64SHDRSIZE /* Must be ELF64SHDRSIZE */
 
 	// 32-bit architectures
-	case sys.ARM:
+	case sys.ARM, sys.MIPS:
+		if SysArch.Family == sys.ARM {
 			// we use EABI on linux/arm, freebsd/arm, netbsd/arm.
 			if Headtype == obj.Hlinux || Headtype == obj.Hfreebsd || Headtype == obj.Hnetbsd {
 				// We set a value here that makes no indication of which
@@ -959,6 +960,9 @@ func Elfinit(ctxt *Link) {
 				// appropriate.
 				ehdr.flags = 0x5000002 // has entry point, Version5 EABI
 			}
+		} else if SysArch.Family == sys.MIPS {
+			ehdr.flags = 0x50000000 /* MIPS 32 */
+		}
 		fallthrough
 	default:
 		ehdr.phoff = ELF32HDRSIZE

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

@@ -546,6 +546,12 @@ func ldelf(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
 		Errorf(nil, "%s: elf %s unimplemented", pn, SysArch.Name)
 		return
 
+	case sys.MIPS:
+		if elfobj.machine != ElfMachMips || hdr.Ident[4] != ElfClass32 {
+			Errorf(nil, "%s: elf object but not mips", pn)
+			return
+		}
+
 	case sys.MIPS64:
 		if elfobj.machine != ElfMachMips || hdr.Ident[4] != ElfClass64 {
 			Errorf(nil, "%s: elf object but not mips64", pn)