Mark sancov test as unsupported on Darwin

This test has been failing on all Darwin bots since it was introduced:
http://lab.llvm.org:8080/green/job/clang-stage1-configure-RA_check/32111

fatal error: error in backend: Global variable '__sancov_gen_' has an invalid section specifier '__DATA,__sancov_counters': mach-o section specifier requires a section whose length is between 1 and 16 characters.
Target: x86_64-apple-darwin15.6.0

llvm-svn: 304673