target/aarch64: Add watchpoint support

target/aarch64: Add watchpoint support

There are some breakpoint/watchpoint related code in armv8_dpm.c,
but seems not working for aarch64. Target aarch64 has its own
breakpoint implementation in aarch64.c. This commit follows the
same logic to add watchpoint support for target aarch64.

This commit also increases the size of stop_reason[] in function
gdb_signal_reply() since the old size is too small to fit in a
64-bit address, such as ffff8000115e6980.

Change-Id: I907dc0e648130e36b434220f570c37d0e8eb5ce1
Signed-off-by: Liming Sun <lsun@mellanox.com>
Signed-off-by: Daniel Goehring <dgoehrin@os.amperecomputing.com>
Reviewed-on: http://openocd.zylin.com/4761
Tested-by: jenkins
Reviewed-by: Liming Sun <limings@nvidia.com>
Reviewed-by: Kevin Burke <kevinb@os.amperecomputing.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>