gdb: allow building on AArch64 systems

gdb has had AArch64 support since 7.6 which is now the default minimum
version of GDB in the tree. Older versions are now legacy.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>