[sanitizer] Fall back to fast unwinder

`-fno-exceptions -fno-asynchronous-unwind-tables` compiled programs don't
produce .eh_frame on Linux and other ELF platforms, so the slow unwinder cannot
print stack traces. Just fall back to the fast unwinder: this allows
-fno-asynchronous-unwind-tables without requiring the sanitizer option
`fast_unwind_on_fatal=1`

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D102046