[ubsan-minimal] Test exported symbol set against RTUBsan

Check that the symbol sets exported by the minimal runtime and the full
runtime match (making exceptions for special cases as needed).

This test uses some possibly non-standard nm options, and needs to
inspect the symbols in runtime dylibs. I haven't found a portable way to
do this, so it's limited to x86-64/Darwin for now.

llvm-svn: 313615