linux-user, bsd-user: disable on unsupported host architectures

Safe signal handling around system calls is mandatory for user-mode
emulation, and requires a small piece of handwritten assembly code.
So refuse to compile unless the common-user/host subdirectory exists
for the host architecture that was detected or selected with --cpu.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>