Partial fix for a bug in rev.1.231. If suspend/resume clobbers the
RTC state, then it may clobber the RTC index register, so the index
register must be restored before using it to restore control registers
in rtc_restore().

The following problems remain:

• rtc_restore() is only called if pmtimer is configured. Buggy suspend/resumes are more likely to clobber the index register than a control register, so pmtimer is more needed than it used to be.
• pmtimer doesn't exist for amd64.
• Restoring of the RTC state may race with rtcintr(). If an RTC interrupt is handled before the state is restored, then rtcin(RTC_INTR) in rtcintr() may read from the wrong register, so rtcintr() may spin forever. This may be mitigated by the most common state clobbering being to turn off RTC interrupts.