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

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

Andreas Gustafsson writes:
> matthew green wrote:
> > i plan to switch amd64 and arm64 to GCC 7 soon.
> 
> The number of ATF test failures on real amd64 hardware jumped
> from 0 (+ occasional random failures) to 44:
> 
>   http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2019/2019.02.08.13.37.46/test.html#failed-tcs-summary

with latest sources i've reduced this to the 3 FP ones and
4 of the others that are still failing for me all related
to c++ call_once stuff:

    t_call_once:call_once_pic_profile, t_call_once:call_once_pic_profile_32, t_call_once2:call_once2_32, t_call_once2:call_once2_static