kvmtool: Makefile: only enable framebuffer output on x86

The Makefile checks for the libaries needed to display the guest's
framebuffer unconditionally on all architectures.
However currently only x86 supports a guest framebuffer.
Introduce ARCH_HAS_FRAMEBUFFER in the Makefile and check for the
needed libraries only on x86. If other architectures add support for
it later, we can add them easily.
This avoids pulling in unneeded libraries into the lkvm binary, which
breaks copying over binaries into systems with only minimal userland.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>