Modify "crossgen -createpdb" to skip loading clrjit.dll (dotnet/coreclr#6607)

Modify "crossgen -createpdb" to skip loading clrjit.dll (dotnet/coreclr#6607)

The -JITPath option doesn't work properly when CrossGen is used to
create a PDB file (issue dotnet/coreclr#6607). Since clrjit.dll isn't really
needed for creating PDB files, the issue is fixed by modifying
CrossGen to skip loading clrjit.dll when -createpdb option is given.

Also following suggestion from issue dotnet/coreclr#6607, added a new switch
-diasymreaderPath to CrossGen, to allow loading diasymreader.dll
from an alternative path.

Commit migrated from https://github.com/dotnet/coreclr/commit/21df26e038c4039150a7978188ecd0b6a5f23d2b