sci-electronics/iverilog: fix 10.3 gcc-10 build problem

sci-electronics/iverilog: fix 10.3 gcc-10 build problem

gcc-10 and above flipped a default from -fcommon to -fno-common:
https://gcc.gnu.org/PR85678
Usually all it takes is to add a few 'extern' declarations and
move definitions from header files to modules. I've port iverilog
to gcc-10.

Closes: https://bugs.gentoo.org/706366
Package-Manager: Portage-2.3.85, Repoman-2.3.20
Signed-off-by: Huang Rui <vowstar@gmail.com>
Closes: https://github.com/gentoo/gentoo/pull/14619
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>