The rdhwr instruction reads synci_step, which is not allowed by OpenBSD
and is not available for ISA older than mips{32/64}r2.
Restrict the rdhwr+synci to Linux on MIPS R6 and fall back to
_flush_cache on all other MIPS targets including OpenBSD and pre-R6
Linux.
Fix#219896.
(cherry picked from commit cff0e6b5dc60cce76e85f0d9e9af537e76759315)