audio/ardour6: update to 6.3.0 and unbreak

attempt to fill in the blanks (like FreeBSD support) in this port's
wscript's CPU feature detection. Build time CPU detection is problematic
in the context of package builds, so for now this assumes that all
amd64 CPUs have SSE and i386 CPUs don't. Juggling the flags in this
way also fixes the breakage uncovered by the latest glib update.

PR: 249959
Approved by: Michael Beer (maintainer)