RHEL 8 support

[omajid]

This is for tracking the status of source-build support on RHEL 8 for release/2.1

Task	PR against master	PR against release/2.1
Add RHEL 8 runtime ids	MERGED	MERGED
Support using python 3 to build coreclr. python and python2 are not present.	MERGED: 1 2 3	TODO
Support using OpenSSL 1.1. OpenSSL 1.1 is the only version of openssl available	MERGED	MERGED

[omajid]

I am going to close this as fixed.

The outstanding python2 vs python3 issues are trivial to patch around. RHEL 8 also has python 2 available in repositories for now (not installed out of the box, though).