eclass/toolchain.eclass: mask LIBPATH for cross-case, bug #692844

eclass/toolchain.eclass: mask LIBPATH for cross-case, bug #692844

/usr/lib/gcc/${CTARGET}/${GCC_CONFIG_VER} contains libraries
destined to be used by ${CTARGET}. revdep-rebuild complains
about missing dependencies against them as we don't populate
LDPATH (or anything else) for them.

The change populates /etc/revdep-rebuild/05cross-${CTARGET}-${GCC_CONFIG_VER}
with a single entry:
    SEARCH_DIRS_MASK="/usr/lib/gcc/${CTARGET}/${GCC_CONFIG_VER}"

crossdev will still own root's SEARCH_DIRS_MASK="/usr/${CTARGET}".

Reported-by: crocket
Closes: https://bugs.gentoo.org/692844
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>