dev-lang/rust: add packages for >=sys-devel/llvm-3.7.0

dev-lang/rust: add packages for >=sys-devel/llvm-3.7.0

Because of changes Gentoo has made with how sys-devel/llvm is built
the way we have to depend on it has changed. LLVM 3.6.0 is only
a build time DEPEND. While LLVM 3.7.0 and newer is a run-time
RDEPEND and DEPEND. Since no EAPI provides a dynamic way for
packages to update RDEPEND we need to provide two different
packages so that people can build against LLVM 3.6.0 and LLVM
3.7.0 and newer.

Package-Manager: portage-2.2.26
Signed-off-by: Doug Goldstein <cardoe@gentoo.org>