Currently there are a lot of platform/compiler specific flags in our mongo_src_rules macros, ex:

https://github.com/10gen/mongo/blob/master/bazel/mongo_src_rules.bzl#L432-L435

Please move as many of the Linux/MacOS specific flags to the toolchain config files as possible:
Linux: https://github.com/10gen/mongo/blob/master/bazel/toolchains/mongo_cc_toolchain_config.bzl 
MacOS: https://github.com/10gen/mongo/blob/master/bazel/toolchains/mongo_apple/mongo_apple_cc_toolchain_config.bzl 

While you're there renaming mongo_cc_toolchain_config to mongo_linux_cc_toolchain_config would be awesome!

For context:

• ppc64le & s390x are linux only, the toolchain config has access to the cpu architecture so you can branch off of that in the toolchain config
• gcc is Linux only, the compiler type is also available in the toolchain config for branching
• clang is on Linux & MacOS, so you'll have to copy the clang options to both toolchain configs in some cases
• Windows is MSVC only, please ignore for the scope of this ticket
• Some options may be harder to move over than others, don't feel like you have to move them all over in one PR. I would start with a PR on the easier ones and then possibly make a followup for the harder ones.