Annotate vfork for hwasan.

Call a hwasan hook in the parent return path for vfork() to let hwasan
update its shadow. See https://github.com/google/sanitizers/issues/925
for more details.

Bug: 112438058
Test: bionic-unit-tests
Change-Id: I9a06800962913e822bd66e072012d0a2c5be453d