Add IBC support to managed build (#10180)

Add IBC support to managed build (#10180)

Add a new build switch, `ibcinstrument`, that adds `/Tuning` on the `crossgen`
command line when building System.Private.CoreLib and its peers. Automatically
consume IBC optdata during builds when these conditions are met:

1. `ibcinstrument` is *not* passed to the build,
2. optdata is available
3. ibcmerge is available

Note that `optdata` will not yet be restored with this change; once packages for
master are made available, a new package reference will still need to be added.
This PR attempts to unblock manually testing IBC, as well as surrounding
CI/infrastructure work.

To help produce an IBC-optimized build using manually generated profile data,
run the newly added `tests/scripts/optdata/bootstrap.py` script. It will
configure the build to consume IBC data from a path automatically, and print out
that path where profile data can be copied.