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>

src/target/riscv/riscv.c

@@ -4128,7 +4128,7 @@ int riscv_init_registers(struct target *target)
 		reg_name += strlen(reg_name) + 1;
 		assert(reg_name < info->reg_names + target->reg_cache->num_regs *
 				max_reg_name_len);
-		r->value = &info->reg_cache_values[number];
+		r->value = info->reg_cache_values[number];
 	}
 
 	return ERROR_OK;