Changes to prepare for aarch64 support

o Generalize support for different architectures.
o Add specific support for aarch64
o Add ALLWINNER64 as a new family using aarch64, but no ports use it yet.
o CROSS_COMPILE is the string to pass to u-boot
o COMPILER is the FreeBSD port (we assume gcc since that works better

typically than clang).

Submitted by: Emmanuel Vadot (the aarch64 bits, hacked by me to be generic)