Changes that improve DTrace FBT reliability

on freebsd/arm64:

- Implement a dtrace_getnanouptime(), matching the existing
  dtrace_getnanotime(), to avoid DTrace calling out to a potentially
  instrumentable function.

  (These should probably both be under KDTRACE_HOOKS.  Also, it's not clear
  to me that they are correct implementations for the DTrace thread time
  functions they are used in .. fixes for another commit.)

- Don't allow FBT to instrument functions involved in EL1 exception handling
  that are involved in FBT trap processing: handle_el1h_sync() and
  do_el1h_sync().

- Don't allow FBT to instrument DDB and KDB functions, as that makes it
  rather harder to debug FBT problems.

Prior to these changes, use of FBT on FreeBSD/arm64 rapidly led to kernel
panics due to recursion in DTrace.

Reliable FBT on FreeBSD/arm64 is reliant on another change from @andrew to
have the aarch64 instrumentor more carefully check that instructions it
replaces are against the stack pointer, which can otherwise lead to memory
corruption.  That change remains under review.

MFC after:	2 weeks
Reviewed by:	andrew, kp, markj (earlier version), jrtc27 (earlier version)
Differential revision:	https://reviews.freebsd.org/D27766

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