arm: make 'MAKEALL -a' distinguish between arm and aarch64

The vexpress_aemv8a is the first aarch64 board in U-Boot.
As it was introduced, it gets built when "MAKEALL -a arm"
is invoked, and fails as this command is run with a 32-bit,
not 64-bit, toolchain as the cross-compiler.

Introduce 'aarch64' as a valid 'MAKEALL -a' argument, treated
as 'arm' for all other intents, and change the architecture
of the vexpress_aemv8a entry in boards.cfg from 'arm' to
'aarch64'.