[compiler-rt] Try to disable failing test on Darwin.

Looks like this test fails on Darwin x86_64 as well:

http://green.lab.llvm.org/green/job/clang-stage1-RA/8593/

Command Output (stderr):
--
fatal error: error in backend: Global variable '__sancov_gen_' has an invalid section specifier '__DATA,__sancov_bool_flag': mach-o section specifier requires a section whose length is between 1 and 16 characters.