arm64: Implement the ./configure --processor option

The help text for the ./configure --processor option says:

    --processor=PROCESSOR  processor to compile for (cortex-a57)

but, unlike arm, the build system does not pass a -mcpu argument to the
compiler. Fix it, and bring arm64 at parity with arm.

Note that this introduces a regression, which is also present on arm: if
the --processor argument is something that the compiler doesn't understand,
but qemu does (like 'max'), then compilation fails. This will be fixed in a
following patch; another fix is to specify a CPU model that gcc implements
by using --cflags=-mcpu=<cpu>.

Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>