README: ports switching to GCC 8.

[matthew green <mrg%eterna.com.au@localhost>]

hi folks.


i've recently switched sparc* to GCC 8 and will be soon
switching powerpc.  just running a few more tests to
ensure builds should work.

you'll probably need to do a clean build, though at least
tools/gcc itself will be OK.

as always, please send-pr or email me with issues related
to this.


.mrg.