there are no kernels currently that do page coloring,
so the only use of cachectl[] is flushing the icache
(on arm and ppc).
on pc64, cachectl consumes 32 bytes in each page resulting
in over 200 megabytes of overhead for 32gb of ram with 4K
pages.
this change removes cachectl[] and adds txtflush ulong
that is set to ~0 by pio() to instruct putmmu() to flush
the icache.
