[System] Bring .NET implementation of System.Text.RegularExpressions

[System] Bring .NET implementation of System.Text.RegularExpressions

This patch brings .NET's implementation of the RegularExpressions
stack and brings some of the supporting infrastructure required to
build without resource files.

The build commands have been updated with two new defines to assist in
bringing .NET code: MONO (to indicate that the code is being compiled
as part of Mono) and a more granular DISABLE_CAS_USE that is used to
mark regions where Code Access Security is being used in the source
code, as Mono does not actually ship with a working CAS implementation.