Log Message:
mozjs78: fix NetBSD i386 builds with Rust >= 1.50

The NetBSD i386 Rust 1.50 builds introduced two different targets (i586
and i686), where previously there was only one (i686). Unfortunately,
the upstream mozjs78 configuration script that narrows to the
appropriate compiler target is confused by this, since it was receiving
"i486" from pkgsrc tooling and didn't know how to pick from more than
one potential approximate match. I'm addressing it this (kludgy) way
for now, as I don't have time to go through Mozilla's scripting and
make adjustments, and I would like this building in the 2021Q2 branch.
If someone else feels there's a better way, please have at it. With
this tweak in place, I'm able to build the xfce4 meta package for
NetBSD/i386 9.2.