sys-devel/kgcc64: drop GCC_FILESDIR override, bug #629776

sys-devel/kgcc64: 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 copies
FILESDIR files used by toolchain.eclass in crosscompilation
mode only (kgcc does not support native builds)

Bug: https://bugs.gentoo.org/629776
Package-Manager: Portage-2.3.8, Repoman-2.3.3