[libc] Add some checks to the mmap wrapper (#197694)

- check that the discarded offset bits (both high and low) are zero
(page alignment is checked in the kernel, but this cannot be done values
we discard for mmap2, nor for truncated values on 32-bit systems)
- check for negative offsets (the kernel interface uses unsigned values,
but our off_t is signed)

One thing I'm not checking, but other implementations do, is the size of
the allocation (after page alignment) fits into a ptrdiff_t. I didn't do
that now as it requires figuring how to get (and whether to cache) the
page size. This is mainly relevant for 32-bit systems as no 64-bit
system will let you allocate 2^63 bytes of (virtual) memory.