amd64, i386: add delay_init(): basic delay(9) implementation management

amd64, i386: add delay_init(): basic delay(9) implementation management

Because the clock situation on x86 and amd64 is a terminal
clusterfuck, there are many different ways to delay(9).  We need a
rudimentary mechanism for gracefully switching to progressively better
delay(9) implementations as they become available during boot without
riddling the code with ifdefs and function pointer comparisons.

This patch adds delay_init() to both amd64 and i386.  If the quality
value passed to delay_init() exceeds the quality value of the current
delay_func, delay_init() changes delay_func to the given function
pointer and updates the quality value.  Both platforms start with
delay_func set to i8254_delay() and a quality value of zero: all other
delay(9) implementations are preferable.

Idea and patch provided by jsg@.  With tons of input, research, and
advice from jsg@.

Link: https://marc.info/?l=openbsd-tech&m=166053729104923&w=2
ok mlarkin@ jsg@