[UBSan] Call UBSan initialization as early as possible.

Specifically, use .preinit_array initialization on Linux and dynamic global
initializer on another platforms. Historically UBSan didn't have any
initialization code and its runtime was stateless. This is no longer the
case - UBSan relies on some non-trivial functionality from sanitizer_common
(e.g. online symbolization) and is now configurable by runtime flags.
Additionally, we've dropped support for enabling UBSan only for a few shared
objects, so UBSan is now always linked into the main executable, so now
we can use similar initialization as all the rest sanitizers.

llvm-svn: 213983