buildman: support newer gcc versions from kernel.org

Add support for gcc versions 7.3.0, 6.4.0 and 4.9.4.

Also use a regex for matching the tarball names. Some gcc versions
use '-ARCH-' instead of '_ARCH-'.

As part of this, we switch TravisCI to also using these toolchains for
all platforms.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>