toolchain.eclass: drop GCC_FILESDIR override, bug #629776

toolchain.eclass: drop GCC_FILESDIR override,  bug #629776

GCC_FILESDIR support was broken by portage-2.3.6 commit:
    https://github.com/gentoo/portage/commit/0637c95545ab7dc0bb5d091de221b9e99e2cf5b2
("portage.package.ebuild: Use a fake FILESDIR to catch invalid accesses")

Unfortunately it was applied to older EAPIs without fixing all
the users first.

The change yanks GCC_FILESDIR override support and hardcodes
FILESDIR usage (as all the ebuilds except 'gcc' are broken anyway).

Bug: https://bugs.gentoo.org/629776
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>