Improve SIGILL support.

Include the illegal instruction in the header if we get a
SIGILL. Otherwise (since these tend to be one-off bit flips), we don't
usually have any information to try to confirm our suspicion that any
given instance is actually a one-off bit flip.

Also add `SIGILL` as a crasher option to easily generate such crashes.

Before:
  signal 4 (SIGILL), code 1 (ILL_ILLOPC), fault addr 0xab1456da

After:
  signal 4 (SIGILL), code 1 (ILL_ILLOPC), fault addr 0xab1456da (*pc=0xe7f0def0)

Bug: http://b/77274448
Test: ran crasher
Change-Id: I5f8dedca5eea2b117b1b1e48430214b38e1366ed