exporters for pkg-config: align with the changes for CMake

The latest CMake exporter changes reworked the the variables in builddata.pm
and installdata.pm.  Unfortunately, the pkg-config exporter templates were
forgotten in that effort.

Fixes #25299

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25308)

(cherry picked from commit 15b74849)