runtime: load runtime.goarm as a byte, not a word

Fixes #6952.

runtime.asminit was incorrectly loading runtime.goarm as a word, not a uint8 which made it subject to alignment issues on arm5 platforms.

Alignment aside, this also meant that the top 3 bytes in R11 would have been garbage and could not be assumed to be setting up the FPU reliably.

R=iant, minux.ma
CC=golang-codereviews
https://golang.org/cl/46240043

src/pkg/runtime/asm_arm.s

@@ -91,7 +91,7 @@ TEXT runtime·breakpoint(SB),NOSPLIT,$0-0
 
 TEXT runtime·asminit(SB),NOSPLIT,$0-0
 	// disable runfast (flush-to-zero) mode of vfp if runtime.goarm > 5
-	MOVW	runtime·goarm(SB), R11
+	MOVB	runtime·goarm(SB), R11
 	CMP	$5, R11
 	BLE	4(PC)
 	WORD	$0xeef1ba10	// vmrs r11, fpscr