[Fuchsia] Create the VMO during initialization, not during exit

We want to avoid doing expensive work during atexit since the process
might be terminated before we can publish the VMO and write out the
symbolizer markup, so move the VMO creation to the initialization
phase and only write data during the atexit phase.

Differential Revision: https://reviews.llvm.org/D66323

llvm-svn: 369180