Don't enter DDB for fatal traps before panic by default.

[jhb]

Summary

Add a new 'debugger_on_trap' knob separate from 'debugger_on_panic'
and make the calls to kdb_trap() in MD fatal trap handlers prior to
calling panic() conditional on this new knob instead of
'debugger_on_panic'. Disable the new knob by default. Developers who
wish to recover from a fatal fault by adjusting saved register state
and retrying the faulting instruction can still do so by enabling the
new knob. However, for the more common case this makes the user
experience for panics due to a fatal fault match the user experience
for other panics, e.g. 'c' in DDB will generate a crash dump and reboot
the system rather than being stuck in an infinite loop of fatal fault
messages and DDB prompts.

Test Plan

• get a panic due to a fatal page fault and try to use 'c'