[build] Use platform-specific -std=c++14 compile flags (#32459)

With the `--copt="-std=c++14"` setting in the bazel.rc file as it is
today, MSVC builds have complained for every cc file:

```
cl : Command line warning D9002 : ignoring unknown option '-std=c++14'
```

This adds thousands of lines of noise to Windows builds, and hides
useful warnings. Using the `/std:c++14` flag on MSVC (and clang-cl) gets
us the desired result.

tools/bazel.rc

@@ -1,10 +1,24 @@
 # bazelrc file
 
+# for platform-appropriate cxxopts
+common --enable_platform_specific_config
+
+build:windows --cxxopt='/std:c++14'
+build:windows --host_cxxopt='/std:c++14'
+build:linux   --cxxopt='-std=c++14'
+build:linux   --host_cxxopt='-std=c++14'
+build:macos   --cxxopt='-std=c++14'
+build:macos   --host_cxxopt='-std=c++14'
+build:freebsd   --cxxopt='-std=c++14'
+build:freebsd   --host_cxxopt='-std=c++14'
+
+# Don't trigger --config=<host platform> when cross-compiling.
+build:android --noenable_platform_specific_config
+build:ios --noenable_platform_specific_config
+
 build --client_env=CC=clang
 build --copt=-DGRPC_BAZEL_BUILD
-build --cxxopt='-std=c++14'
 build --host_copt=-DGRPC_BAZEL_BUILD
-build --host_cxxopt='-std=c++14'
 build --action_env=GRPC_BAZEL_RUNTIME=1
 build --define=use_fast_cpp_protos=true