target: disable armv6m unaligned memory access

Change-Id: I42704cf80939ab9c9d4f402d2cd51c196e2fadb3
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/645
Tested-by: jenkins

src/target/arm.h

@@ -125,6 +125,9 @@ struct arm {
 	/** Flag reporting unavailability of the BKPT instruction. */
 	bool is_armv4;
 
+	/** Flag reporting armv6m based core. */
+	bool is_armv6m;
+
 	/** Flag reporting whether semihosting is active. */
 	bool is_semihosting;