target/register: use an array of uint8_t for register's value

The use of 'void *' makes the pointer arithmetic incompatible with
standard C, even if this is allowed by GCC extensions.
The use of 'void *' can also hide incorrect pointer assignments.

Switch to 'uint8_t *' and add GCC warning flag to track any use of
pointer arithmetic extension.

Change-Id: Ic4d15a232834cd6b374330f70e2473a359b1607f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5937
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

configure.ac

@@ -833,6 +833,7 @@ AS_IF([test "x${gcc_wextra}" = "xyes"], [
   GCC_WARNINGS="${GCC_WARNINGS} -Wbad-function-cast"
   GCC_WARNINGS="${GCC_WARNINGS} -Wcast-align"
   GCC_WARNINGS="${GCC_WARNINGS} -Wredundant-decls"
+  GCC_WARNINGS="${GCC_WARNINGS} -Wpointer-arith"
 ])
 AS_IF([test "x${gcc_werror}" = "xyes"], [
   GCC_WARNINGS="${GCC_WARNINGS} -Werror"