Cross-DSO control flow integrity (compiler-rt part).

This is an initial version of the runtime cross-DSO CFI support
library.

It contains a number of FIXMEs, ex. it does not support the
diagnostic mode nor dlopen/dlclose, but it works and can be tested.
Diagnostic mode, in particular, would require some refactoring (we'd
like to gather all CFI hooks in the UBSan library into one function
so that we could easier pass the diagnostic information down to
__cfi_check). It will be implemented later.

Once the diagnostic mode is in, I plan to create a second test
configuration to run all existing tests in both modes. For now, this
patch includes only a few new cross-DSO tests.

llvm-svn: 255695