machine/arch-arm64: add -mbranch-protection=standard

Enable branch protection (PAC/BTI) for all aarch64 builds.  This was
previously enabled at a global level in the GCC build, but that breaks
the gcc test suite.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8905639d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>