makefiles: Build jailhouse-{amd,intel} together

makefiles: Build jailhouse-{amd,intel} together

This build system update brings an ability to build "jailhouse.bin"
firmware for different architecture "variants" (currently, AMD and
Intel on x86_64) simultaneously. Corresponding firmware files are
called 'jailhouse-$variant.bin'. Variants are introduced in
$(BUILD_VARIANTS) in hypervisor/Makefile and also in corresponding
arch Makefiles. Having per-variant compilation options inside one
architecture is not supported.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>