autoconf: more variants for OSARCH linux-gnu

There are quite a few odd GNU/Linux platforms. Just call all of them
linux-gnu.

Specifically this fixes building the Debian platforms mips64el and x32.
And maybe also others.

ASTERISK-26546 #close

Change-Id: I06ec4bd7f0ee1c84b6b24d81538223b07c4174b1