linux-user: fix mmap_find_vma_reserved()

linux-user: fix mmap_find_vma_reserved()

The value given by mmap_find_vma_reserved() is used with mmap(),
so it is needed to be aligned with the host page size.

Since commit 18e80c55bb, reserved_va is only aligned to TARGET_PAGE_SIZE,
and it works well if this size is greater or equal to the host page size.

But ppc64 hosts have 64kB page size and when we start a 4kiB page size
guest (like i386), it fails when it tries to mmap the stack:

    mmap stack: Invalid argument

Fixes: 18e80c55bb (linux-user: Tidy and enforce reserved_va initialization)
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180714193553.30846-1-laurent@vivier.eu>