Please answer these questions before submitting your issue.

Should this be an issue in the gRPC issue tracker?

Yes, for tracking reasons

What version of gRPC and what language are you using?

1.9.0-dev

What operating system (Linux, Windows, …) and version?

MacOS

What runtime / compiler are you using (e.g. python version or version of gcc)

$ bazel info
release: release 0.8.1-homebrew
$ clang -v
Apple LLVM version 9.0.0 (clang-900.0.38)

What did you do?

bazel build //:grpc

What did you expect to see?

Successful build as was possible before exec_ctx became a thread-local variable

What did you see instead?

$ bazel build //:grpc
INFO: Analysed target //:grpc (17 packages loaded).
INFO: Found 1 target...
ERROR: /Users/vpai/Git/grpc/BUILD:224:1: Linking of rule '//:grpc' failed (Exit 1)
clang: warning: argument unused during compilation: '-pthread' [-Wunused-command-line-argument]
ld: illegal thread local variable reference to regular symbol __ZN9grpc_core7ExecCtx9exec_ctx_E for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Anything else we should know about your project / environment?

A similar issue is reported (and unresolved) at tensorflow/serving#1 and other places in Github