Indicate userfaultfd in application threads' start function

In order to identify if we are using userfaultfd GC or not, use a
wrapper around Thread::CreateCallback, which is the start function for
mutator threads, to indicate that. This will be helpful in identifying
the GC type by looking at stack traces.

Bug: 160737021
Test: manual
Change-Id: I8cb3c4871d2ef4ae61f6f3fddd8ea4b0a57c5aa8