Fix crossgen debug directory generation problems. (#12715)

Fix crossgen debug directory generation problems. (#12715)

* Fix crossgen debug directory generation problems.

The first problem was that when the existing/incoming PDB debug
directory entry was a portable PDB (MinorVersion == 0x504d), the
ngen/native PDB added had the same MinorVersion indicating that
it was a portable PDB (but it never can be).

This was fixed by setting MinorVersion to 0 when creating the
ngen PDB debug directory entry.

The second problem was that the ngen PDB entry was being created
even when crossgen was run on linux/mac, etc.

The fix was to ifdef NO_NGENPDB the save ngen PDB entry code.