[ubsan] Fall back to the fast unwinder when print_stacktrace=1

This makes it possible to get stacktrace info when print_stacktrace=1 on
Darwin (where the slow unwinder is not currently supported [1]). This
should not regress any other platforms.

[1] The thread about r300295 has a relatively recent discusion about
this. We should be able to enable the existing slow unwind functionality
for Darwin, but this needs more testing.

Differential Revision: https://reviews.llvm.org/D32517

llvm-svn: 301839