gdb_server: add support for architecture element

gdb_server: add support for architecture element

This change adds optional support for a target to report architecture
information in the target description to GDB. This is needed by some GDB
implementations to properly support remote target with custom behavior.
More information on the architecture element can be found here:

    https://sourceware.org/gdb/onlinedocs/gdb/Target-Description-Format.html#Target-Description-Format

Change-Id: I57b19cae5ac3496256e4e5cc52cf6526ca5c322d
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/4078
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>