toolchain.eclass: update active cross-gcc on uninstall

toolchain.eclass: update active cross-gcc on uninstall

gcc-config-2 changed /usr/bin/gcc from a binary to symlink
in commit b907ef80bc421df23515afc4c306e4d96c67649
("Convert binary wrapper to a symlink wrapper.")

Before the above change /usr/bin/gcc worked in both cases
when active cross-gcc is being uninstalled. After the above
change symlink becomes dangling.

The change runs `gcc-config` more aggressively.

Reported-by: Pietro
Closes: https://bugs.gentoo.org/680360
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>