Use pkg-config.multiarch file to determine architecture

The file contains a GNU Target Triplet string that describes the
sysroot. It is needed for the cross-compilation step so can safely
assume to be available.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4916