armv8: check the core state to pass the correct arch to gdb

armv8: check the core state to pass the correct arch to gdb

Commit 3799eded6761 ("target/aarch64: add support for
multi-architecture gdb") passes the constant string "aarch64" as
architecture to gdb. This is not working if the core is running
in 32 bits mode; gdb reports:
	Truncated register 8 in remote 'g' packet
then closes the connection with OpenOCD.

Make the architecture string dependant from the current state of
the core.

Change-Id: I16e1614ea02ba29bf87f450b3dfe25c83c9a3612
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5234
Tested-by: jenkins
Reviewed-by: Muhammad Omair Javaid <omair.javaid@linaro.org>