gcc & libgcc 6/7/8

[Michael Dickens]

I recently noticed that gcc7 had an update, and so I've started updating
it. I'll move it to the 7.1 release shortly. There's also a new gcc8
snapshot that I'll look into getting up for folks on the bleeding edge
to try out (as I'm often asked to do for UHD / Volk / GNU Radio).

The question comes up as to what version "libgcc" should be. Right now
libgcc is a subport of gcc6. gcc7 requires an updated version of libgcc,
as I'm guessing gcc8 does.

I can easily leave libgcc attached to gcc6, and then disable
libgcc-devel in favor of libgcc7 and libgcc8 ... but, I'm pretty new at
the gcc / libgcc combo so I'm really not sure what the best or proper
way to go here is.

Looking for thoughts and direction on how to proceed. Thx! - MLD