target/aarch64: add support for multi-architecture gdb

target/aarch64: add support for multi-architecture gdb

GDB can be built for multi-architecture through the command
	./configure --enable-targets=all && make
Such multi-architecture GDB requires the target's architecture to
be selected either manually by the user through the GDB command
"set architecture" or automatically by the target description sent
by the remote target (i.e. OpenOCD).

Commit e65acd889c61a424c7bd72fdee5d6a3aee1d8504 ("gdb_server: add
support for architecture element") already provides the required
infrastructure to support multi-architecture gdb.

aarch64-linux-gnu-gdb 8.2 uses "aarch64" as default architecture,
but also supports the value "aarch64:ilp32" and all the values
supported by arm-none-eabi-gdb.
These values can be displayed on arm gdb prompt by typing
"set architecture " followed by a TAB for autocompletion.

Set the gdb architecture value for aarch64 target to "aarch64".

Change-Id: I63e9769f47d8e73f048eb84fa73e082dd1c8e52c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4755
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>