arm64: fix 32 bit memory operations (#63)

Ensure to use unsigned memory move instructions when moving 32 bit
data in a 64 bit register.

If the MSb of a 32 bit word in the memory is 1 the value is interpreted
as signed and stored as such in the 64 bit register.

E.g. moving the 0xe8000020 value from memory to register would result
such register being set to 0xffffffffe8000020. A successive access to
the memory based on such value will most probably trigger a memory
exception.

Author: andrejro

internal/reg/reg_arm64.s

@@ -7,25 +7,25 @@
 
 // func Move(dst uint32, src uint32)
 TEXT ·Move(SB),$0-8
-	MOVW	dst+0(FP), R0
-	MOVW	src+4(FP), R1
+	MOVWU	dst+0(FP), R0
+	MOVWU	src+4(FP), R1
 
 	// copy src to dst
-	MOVW	(R1), R3
-	MOVW	R3, (R0)
+	MOVWU	(R1), R3
+	MOVWU	R3, (R0)
 
 	// zero out src
-	MOVW	$0, R3
-	MOVW	R3, (R1)
+	MOVWU	$0, R3
+	MOVWU	R3, (R1)
 
 	RET
 
 // func Write(addr uint32, val uint32)
 TEXT ·Write(SB),$0-8
-	MOVW	addr+0(FP), R0
-	MOVW	val+4(FP), R1
+	MOVWU	addr+0(FP), R0
+	MOVWU	val+4(FP), R1
 
-	MOVW	R1, (R0)
+	MOVWU	R1, (R0)
 
 	RET