Dump threads in tombstone fallback path.

When dumping a tombstone using the fallback path, only the main
thread was showing up. Modify the code to dump the threads using
a slightly different path for the tombstone generation code.

In addition, while looking at this code, two MTE variables were
not set in the tombstone fallback code. Added those variables
so MTE devices will work properly in this fallback path.

Modified the tombstone unit tests for seccomp to have
multiple threads and verify those threads show up in the tombstone.

Bug: 208933016

Test: Ran unit tests.
Test: Ran debuggerd <PID> on a privileged process and verified
Test: all threads dumped. Also verified that the tagged_addr_ctrl
Test: variable is present on the raven device.
Change-Id: I16eadb0cc2c37a7dbc5cac16af9b5051008b5127