toolchain.eclass: update to EAPI=5 standards

toolchain.eclass: update to EAPI=5 standards

This eclass is inherited by ebuilds in sys-devel/{gcc,kgcc64,gcc-apple},
each which make use of different IUSE flags.   This causes problems with
`use X` constructions when X is not in the IUSE flags.  At EAPI=4 this
simply emitted a warning, while at EAPI=5 this is an error.  We update
the eclass to make use of use_if_iuse and similar constructions where
necessary to bring the eclass into compliance with EAPI=5.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>