Fails compilation with emscripten if host platfrom is Mac

In ca3f87560a0eef716195cadf66dc6b938a579ec6 the following lines were added
https://github.com/abseil/abseil-cpp/blob/master/absl/time/internal/cctz/BUILD.bazel#L80-L82

This causes emscripten builds that execute on Mac as host platform to fail with
```
shared:ERROR: Foundation: No such file or directory ("Foundation" was expected to be an input file, based on the commandline arguments provided)
```
This makes sense, as emscripten does not have OSX Foundation libraries.

To the best of my knowledge CROSSTOOL in bazel, while allowing to change `cpu`, does not allow to change `os`.