Run IBCMerge step for Linux on Windows (#20496)

Run IBCMerge step for Linux on Windows (#20496)

This change does the following:

* Move the IBCOptimize step out of the Crossgen section and into the CoreLib build section of build.cmd
* Adds -ibconly which will skip building System.Private.CoreLib and only run the ibcmerge step
* Adds crossgenonly and partialngen flags to build.sh

These three changes facilitate our ability to apply IBC data to Linux assemblies on Windows and the perform the crossgen step on Linux, which will be our flow for official builds when we want to apply IBC data since IBCMerge cannot run on non-Windows platforms