gdb: enable host-gdb on aarch64

gdb has full support for aarch64 since version 7.6,
current minimum is 7.7.1.
Target build was re-enabled over a year ago, but host-gdb was not.

Related commits:
0e49733b gdb disabled for aarch64
4372449a target-gdb re-enabled

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>