util: move qemu_real_host_page_size/mask to osdep.h

util: move qemu_real_host_page_size/mask to osdep.h

These only depend on the host and therefore belong in the common
osdep, not in a target-dependent object.

While at it, query the host during an init constructor, which guarantees
the page size will be well-defined throughout the execution of the program.

Suggested-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>