README: gcc 7 switch coming to a port near you!

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

hi folks.


i plan to switch amd64 and arm64 to GCC 7 soon.  i386, sparc*,
mips, ppc, and alpha are probably ready and tested enough for
anyone else to try out.  32 bit arm is only just now working
so not well tested yet.  hppa, m68k, vax, and sh3 all build
but have not been tested yet.  ia64 and ppc64 are currently
not building, and i haven't looked at the hopefully revived
riscv port yet, or the or1k.

if you'd like to test now from -current, build a clean tree
with build.sh -V HAVE_GCC=7.  it should just work..

if you need to switch back to GCC 6 afterwards, you'll need
to set HAVE_GCC=6 and build a clean install.


.mrg.