mono6: Disable boehm garbage collector on aarch64

The embedded copy of boehm-gc fails to build on aarch64 and sgen
is considered the "modern" mono garbage collector, so we can
easily build with just that.

For this to work, the arguments need to register with the configure
script properly, which wasn't happening because autogen was running
configure. Stop that from happening.

Bump PKGREVISION

(nia)