Always use LLD on darwin

[dzbarsky]

It sounds like a few folks have tried the -fuse-ld=lld option and it works for them, so let's try to roll it out more broadly.

[dzbarsky]

@rrbutani are you the right person to take a look at this? I tried to keep the cmd lines generated the same as we did before this change (when running with --linkopt=-fuseld=ld64.lld) with the exception of the link sections, but perhaps we can change some of the flags for non-xcompile to the lld variants and simplify things further?

[garymm]

Can add this to the PR description: Fixes: https://github.com/bazel-contrib/toolchains_llvm/issues/212.

would be great to see this merged!

[fmeum]

@dzbarsky Happy to merge after conflicts have been resolved