provide lld on osx and linux for a completely provided native toolchain, and consider a testing process for the osx toolchain

[cosmicexplorer]

See #5640, where @jsirois informs me of the existence of lld. We should add that to the OSX platform-specific subsystems from #5490 and then rip out XCodeCLITools. This allows us to avoid requiring the user to install anything themselves and allows us to roll out security, performance, or functionality upgrades instead of waiting for Apple to push out their own version. It's also supposed to be pretty fast.

We should probably provide lld for Linux as well. Regardless, we might want to run at least a unit test in an OSX shard on Travis if we can provide the whole OSX toolchain. We might also want to consider some sort of CI or at least informal testing for pantsbuild/binaries. This FIXME was left in #5490.

[cosmicexplorer]

Unless we want to start adding symlinks for e.g. cc or ld in our packaged archives that weren't there before, we probably need #5661 to use this in python_dist().

[cosmicexplorer]

I believe the above is actually incorrect and setting LDSHARED will allow use of an arbitrary linker (as in, I just did it while setting LDSHARED=ld64.lld on my osx box and it worked). Will attack the binaries section of this first -- will probably need to change the name of the clang binary to llvm again, because lld would need to be built along with clang and the rest of llvm in a single cmake/make invocation. Will link the pantsbuild/binaries PR here when that is made.

[Eric-Arellano]

Closing as stale, until we add back the native backend.