[clang/Darwin] Remove __llvm_profile_counter_bias from export list for profiling

Do not export __llvm_profile_counter_bias when profiling is enabled
because this symbol is hidden and cannot be exported.

Should fix this bot error:

```
URL: http://green.lab.llvm.org/green/job/clang-stage1-RA/5678/consoleFull

Problem: Command Output (stdout):
--
ld: warning: cannot export hidden symbol ___llvm_profile_counter_bias
from
/Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build/lib/clang/11.0.0/lib/darwin/libclang_rt.profile_osx.a(InstrProfilingBiasVar.c.o)
ld: warning: cannot export hidden symbol ___llvm_profile_counter_bias
from
/Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build/lib/clang/11.0.0/lib/darwin/libclang_rt.profile_osx.a(InstrProfilingBiasVar.c.o)
```