Revert "[CMake] Fix the value of `config.target_cflags` for non-macOS Apple"

This reverts commit 1bcdbd68616dc7f8debe126caafef7a7242a0e6b.

It's been reported that some bots are failing with this change with CMake
error like:

```
CMake Error at /b/s/w/ir/k/llvm-project/compiler-rt/cmake/config-ix.cmake:177 (message):
  Unsupported architecture: arm64
Call Stack (most recent call first):
  /b/s/w/ir/k/llvm-project/compiler-rt/cmake/config-ix.cmake:216 (get_target_flags_for_arch)
  /b/s/w/ir/k/llvm-project/compiler-rt/test/tsan/CMakeLists.txt:78 (get_test_cflags_for_apple_platform)
```

I'm reverting the patch now to unbreak builds. I will investigate properly when time permits.

rdar://problem/50124489

llvm-svn: 359327