[UBSan] Introduce UBSAN_OPTIONS environment variable.

If UBSan is run in a standalone mode (w/o any other sanitizer), it
still uses functions from sanitizer_common, some of which depend on
the value of runtime flags. Allow to override the default values of these
flags with UBSAN_OPTIONS variable. In particular, UBSAN_OPTIONS=symbolize=0
can be used to turn off online symbolization.

llvm-svn: 213782