tools/build/cross-build: Don't include sys/uio.h from linux limits.h

This creates a circular dependency for OpenZFS's libspl in sys/uio.h,
and it shouldn't be needed since the system limits.h already defines
IOV_MAX, so delete it, and unconditionally assert that to be the case.
Otherwise the re-include of libspl's sys/uio.h tries to use PAGESIZE
before it has been defined by OpenZFS's own sys/param.h.

Fixes: 7a7741af18d6 ("zfs: merge openzfs/zfs@b10992582")
MFC after: 1 week

(cherry picked from commit 0bdf2535d4f8a571cb6d230f2f19eb20a452a4f8)