This allows 32-bit packages to be found when the user has specified
PKG_CONFIG_PATH for some other reason.

This also mirrors the way e.g. i686-linux-gnu-pkg-config is implemented on
Debian, and possibly other distributions as well.

This also prevents 64-bit .pc files from being found. This was originally
intended as a benefit [1], but can contribute to misdetection of headers which
are not actually multiarch (e.g. GStreamer, although at the time that [1] was
written that was a preëxisting problem). In general a distribution which
provides .pc files for one architecture should be expected to provide them for
any architecture that it actually provides libraries for; even if that was not
true of Debian in 2017, it is now. I moreover assert it is better to fail to
find a present library than to incorrectly find the wrong one.

Note that we can't easily use i686-linux-gnu-pkg-config, as would otherwise be
preferable, for reasons also described in [1].

[1] https://www.winehq.org/pipermail/wine-devel/2017-June/118002.html