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?

[dzbarsky]

this comment is preexisting but I don't think it's correct?

[rrbutani]

I think it actually might be right because of this bit here:

toolchains_llvm/toolchain/cc_toolchain_config.bzl

Lines 188 to 189 in 6edc43b

	if stdlib == "builtin-libc++" and is_xcompile:
	stdlib = "stdc++"

i.e. builtin-libc++ implies not is_xcompile, and this arm has exec_os != "darwin" implying linux (exec) -> linux (target)

[rrbutani]

definitely no objection to rewording that comment though

[rrbutani]

Apologies for the delay — unfortunately I haven't really kept up with the state of lld on darwin and no longer have a machine to test this on.

However, everything in this PR looks good to me. Appreciate the cleanup as well.

[rrbutani]

I think it actually might be right because of this bit here:

toolchains_llvm/toolchain/cc_toolchain_config.bzl

Lines 188 to 189 in 6edc43b

	if stdlib == "builtin-libc++" and is_xcompile:
	stdlib = "stdc++"

i.e. builtin-libc++ implies not is_xcompile, and this arm has exec_os != "darwin" implying linux (exec) -> linux (target)

[rrbutani]

definitely no objection to rewording that comment though