linux-user: Align mmap_find_vma to host page size

This can avoid stack allocation failures for i386 guest
on ppc64 (64k page) host.

Suggested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190519201953.20161-14-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>