dev-libs/libffi-compat: new package, provides libffi.so.6

dev-libs/libffi-compat: new package, provides libffi.so.6

There are still a few binary-only packages out there that rely on
'libffi.so.6' presence. This package provides these libraries.
No development headers here. Thus this library can co-exist with
dev-libs/libffi:0/7 package.

Ebuild is based on dev-libs/libffi. We can fast-stabilize it if
in-tree stable packages do rely on libffi.so.6 (they should not).

The patch is almost entirely by Timo Rothenpieler.
I renamed a package and made a few minor tweaks.

Fix-by: Timo Rothenpieler
Reported-by: Timo Rothenpieler
Closes: https://bugs.gentoo.org/695964
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>