Fix parallel make of OpenSSL

Summary:
It's been explicitly forcing -j1 instead of allowing it to build in parallel. Looks like this has been broken since the initial commit. Explicitly specifying the *.pc targets is what breaks the parallel build. If they're omitted they'll get built anyway, in proper order.

This is a backport of https://github.com/monero-project/monero/pull/7911

Test Plan:
Before:

$ time HOST=x86_64-linux-gnu make openssl
...
real	0m41,339s
user	0m38,485s
sys	0m4,357s

After:

$ time HOST=x86_64-linux-gnu make openssl
real	0m9,713s
user	1m3,751s
sys	0m8,712s

Run gitian builds on CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14037