[ASan] Add a test for __asan_gen_ globals labels on Darwin.

Per Nick Kledzik (http://llvm.org/bugs/show_bug.cgi?id=17976):
"""
For MacOSX, the labels in __cstring section should be 'L' labels, and the labels
in the __const section should be non-L or 'l' labels.  The later ('l') will cause
the linker to remove the label in the final executable.
"""

, so we need to ensure that the globals emitted by ASan have appropriate labels.