Make a few JIT interface changes to accommodate CoreRT:

Make a few JIT interface changes to accommodate CoreRT:
- Add support for providing extended flags for a compile invocation. In order to preserve binary compatibility with older definitions of ICorJitCompiler, this involved adding a new method to ICJI to fetch the flags. This method is only called if no flags are provided to the compiler via compileMethod.
- Add helper definitions for CORINFO_HELP_PINVOKE_{BEGIN,END} for CoreRT
- Add a flag to CorJitFlag2 to indicate that the aforementioned heleprs should be used instead of generating inline transitions
- Add a new method, getAddressOfPInvokeTarget, to the JIT/EE interface. This method can return a direct target for a PInvoke call in addition to an indirect or double-indirect target.

[tfs-changeset: 1572442]