Hidden visibility for non-API functions in Regal and RegalW builds

Before:
-rwxrwxr-x 1 nigels nigels 5.8M Aug 16 14:10 lib/linux/libRegal.so.1.0.0
-rwxrwxr-x 1 nigels nigels 3.1M Aug 16 14:12 lib/linux/libRegalW.so.1.0.0

After:
-rwxrwxr-x 1 nigels nigels 5.5M Aug 16 14:12 lib/linux/libRegal.so.1.0.0
-rwxrwxr-x 1 nigels nigels 3.1M Aug 16 14:36 lib/linux/libRegalW.so.1.0.0

The other benefit is that RegalW is deemed API compatible with Regal by abi-compliance-checker.