Wait 100 microseconds for a local APIC to dispatch each startup-related IPI
rather than 20. The MP 1.4 specification states in Appendix B.2:

"A period of 20 microseconds should be sufficient for IPI dispatch to
 complete under normal operating conditions".

(Note that this appears to be separate from the 10 millisecond (INIT) and
200 microsecond (STARTUP) waits after the IPIs are dispatched.) The
Intel SDM is silent on this issue as far as I can tell.

At least some hardware requires 60 microseconds as noted in the PR, so
bump this to 100 to be on the safe side.

PR: 197756
Reported by: zaphod@berentweb.com
MFC after: 1 week