arch: um: added shim to trap to allow installing a fault catcher for tests

Added context to current thread that allows a test to specify that it
wants to skip the normal checks to run an installed fault catcher.

Change-Id: Ieedf354df2d0d8c924666544e280f43e621f40d1
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>