dev-util/electron: Fix build issues

dev-util/electron: Fix build issues

Fix libvpx build issues by pinning the dependency and adding
system-libvpx USE-flag.

Pin dev-util/gn version, as newer GN seems to be breaking the
build.

Fix build on non-x86, courtesy of Marcus Comstedt.

Closes: https://bugs.gentoo.org/688426
Closes: https://bugs.gentoo.org/679492
Closes: https://bugs.gentoo.org/686984
Signed-off-by: Elvis Pranskevichus <elvis@magic.io>
Package-Manager: Portage-2.3.66, Repoman-2.3.11
Signed-off-by: Joonas Niilola <juippis@gentoo.org>