[libsanitizer] Run the callback on a separate stack in StopTheWorld.

Currently the callback runs on the caller's stack. If this stack
contains values that have gone out of scope, and we are not super careful, those
values can propagate into global variables (the libc sigaction() in particular
has a side effect that can lead to this). This has caused false negatives in
leak checking code.

Changes: map a separate stack space for the tracer thread. Also, move some
globals into local scope (they had no business being global anyway).

Patch by Sergey Matveev (earthdok@google.com)