runtime: arm: abort if hardware floating point missing

Fixes #3911.

Requires CL 6449127.

dfc@qnap:~$ ./runtime.test
runtime: this CPU has no floating point hardware, so it cannot run
this GOARM=7 binary. Recompile using GOARM=5.

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6442109

src/pkg/runtime/asm_arm.s

@@ -37,6 +37,7 @@ TEXT _rt0_arm(SB),7,$-4
 	MOVW.NE	g, R0 // first argument of initcgo is g
 	BL.NE	(R2) // will clobber R0-R3
 
+	BL	runtime·checkgoarm(SB)
 	BL	runtime·check(SB)
 
 	// saved argc, argv

src/pkg/runtime/signal_linux_arm.c

@@ -147,9 +147,21 @@ runtime·setsig(int32 i, void (*fn)(int32, Siginfo*, void*, G*), bool restart)
 #define AT_PLATFORM	15 // introduced in at least 2.6.11
 #define AT_HWCAP	16 // introduced in at least 2.6.11
 #define AT_RANDOM	25 // introduced in 2.6.29
+#define HWCAP_VFP	(1 << 6)
 static uint32 runtime·randomNumber;
-uint32 runtime·hwcap;
 uint8  runtime·armArch = 6;	// we default to ARMv6
+uint32 runtime·hwcap;	// set by setup_auxv
+uint8  runtime·goarm;	// set by 5l
+
+void
+runtime·checkgoarm(void)
+{
+	if(runtime·goarm > 5 && !(runtime·hwcap & HWCAP_VFP)) {
+		runtime·printf("runtime: this CPU has no floating point hardware, so it cannot run\n");
+		runtime·printf("this GOARM=%d binary. Recompile using GOARM=5.\n", runtime·goarm);
+		runtime·exit(1);
+	}
+}
 
 #pragma textflag 7
 void