runtime, cmd/ld, cmd/5l, run.bash: enable external linking on FreeBSD/ARM.

Update #7331

LGTM=dave, iant
R=golang-codereviews, dave, gobot, iant
CC=golang-codereviews
https://golang.org/cl/89520043

src/cmd/5l/obj.c

@@ -62,6 +62,7 @@ archinit(void)
 			sysfatal("cannot use -linkmode=external with -H %s", headstr(HEADTYPE));
 		break;
 	case Hlinux:
+	case Hfreebsd:
 		break;
 	}
 

src/cmd/ld/elf.c

@@ -55,8 +55,8 @@ elfinit(void)
 
 	// 32-bit architectures
 	case '5':
-		// we only use EABI on linux/arm
-		if(HEADTYPE == Hlinux)
+		// we use EABI on both linux/arm and freebsd/arm.
+		if(HEADTYPE == Hlinux || HEADTYPE == Hfreebsd)
 			hdr.flags = 0x5000002; // has entry point, Version5 EABI
 		// fallthrough
 	default:

src/pkg/runtime/rt0_freebsd_arm.s

@@ -11,3 +11,8 @@ TEXT _rt0_arm_freebsd(SB),NOSPLIT,$-4
 	MOVW	$4(R13), R1		// argv
 	MOVM.DB.W [R0-R1], (R13)
 	B	_rt0_go(SB)
+
+TEXT main(SB),NOSPLIT,$-4
+	MOVM.DB.W [R0-R1], (R13)
+	MOVW	$_rt0_go(SB), R4
+	B		(R4)

src/run.bash

@@ -127,13 +127,18 @@ darwin-386 | darwin-amd64)
 	*) go test -ldflags '-linkmode=external'  || exit 1;;
 	esac
 	;;
-dragonfly-386 | dragonfly-amd64 | freebsd-386 | freebsd-amd64 | linux-386 | linux-amd64 | linux-arm | netbsd-386 | netbsd-amd64)
+dragonfly-386 | dragonfly-amd64 | freebsd-386 | freebsd-amd64 | freebsd-arm | linux-386 | linux-amd64 | linux-arm | netbsd-386 | netbsd-amd64)
 	go test -ldflags '-linkmode=external' || exit 1
 	go test -ldflags '-linkmode=auto' ../testtls || exit 1
 	go test -ldflags '-linkmode=external' ../testtls || exit 1
 	
 	case "$GOHOSTOS-$GOARCH" in
 	netbsd-386 | netbsd-amd64) ;; # no static linking
+	freebsd-arm) ;; # -fPIC compiled tls code will use __tls_get_addr instead
+	                # of __aeabi_read_tp, however, on FreeBSD/ARM, __tls_get_addr
+	                # is implemented in rtld-elf, so -fPIC isn't compatible with
+	                # static linking on FreeBSD/ARM with clang. (cgo depends on
+			# -fPIC fundamentally.)
 	*)
 		go test -ldflags '-linkmode=external -extldflags "-static -pthread"' ../testtls || exit 1
 		;;