Also define THR_PAGE_SIZE_MIN for mips

Commit 6173c7833be4, which cherry-picked commit c7904405a8d4 ("Remove
PAGE_SIZE from libthr") to stable/13 resulted in build errors on mips:

/usr/src/lib/libthr/thread/thr_barrier.c:40:50: error: use of undeclared identifier 'THR_PAGE_SIZE_MIN'
_Static_assert(sizeof(struct pthread_barrier) <= THR_PAGE_SIZE_MIN,

^

This is because c7904405a8d4 defined THR_PAGE_SIZE_MIN for all supported
architectures in main/14-CURRENT, but that list no longer includes mips.
Add a similar define for mips, so libthr can build successfully again.

Direct commit to stable/13, since main no longer has mips.

Fixes: 6173c7833be4