meson: Disallow 64-bit on 32-bit HVF/NVMM/WHPX emulation

meson: Disallow 64-bit on 32-bit HVF/NVMM/WHPX emulation

Require a 64-bit host binary to spawn a 64-bit guest.

For HVF this is trivially true because macOS 11 dropped
support for 32-bit applications entirely.

For NVMM, NetBSD only enables nvmm on x86_64:
  http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/dev/nvmm/Makefile?rev=1.1.6.2;content-type=text%2Fplain

For WHPX, we have already dropped support for 32-bit Windows.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>