[dev.cc] runtime: fix arm5 build

TBR=r
CC=golang-codereviews
https://golang.org/cl/168600043

src/runtime/vlop_arm.s

@@ -101,7 +101,7 @@ TEXT _sfloat(SB), NOSPLIT, $68-0 // 4 arg + 14*4 saved regs + cpsr + return valu
 // load the signal fault address into LR, and jump
 // to the real sigpanic.
 // This simulates what sighandler does for a memory fault.
-TEXT _sfloatpanic(SB),NOSPLIT,$-4
+TEXT runtime·_sfloatpanic(SB),NOSPLIT,$-4
 	MOVW	$0, R0
 	MOVW.W	R0, -4(R13)
 	MOVW	g_sigpc(g), LR