runtime: Fix build on older ARM

        The ARM external linking CL used BLX instructions in gcc assembler. Replace with BL to retain support on older ARM processors.

R=rsc
CC=golang-dev
https://golang.org/cl/12938043

src/pkg/runtime/cgo/gcc_arm.S

@@ -25,8 +25,8 @@ EXT(crosscall_arm2):
 	mov r5, r1
 	mov r0, r2
 	mov r1, r3
-	blx r5 // setmg(m, g)
-	blx r4 // fn()
+	bl r5 // setmg(m, g)
+	bl r4 // fn()
 	pop {r4, r5, r6, r7, r8, r9, r10, r11, ip, pc}
 
 .globl EXT(__stack_chk_fail_local)