Only pass -F flag to linker if it is supported

[arichardson]

Summary

LLD only got support for the -F flag in LLVM r308167 so we shouldn't
be using the flag unconditionally.

Test Plan

I wasn't able to build libdl with LLD 4.0 without this change, now it works.

[jhb]

Hmm, lib/Makefile only recurses into libdl if the linker supports filters (that was in the original commit that added libdl: rS320872), so this shouldn't be needed? The libdl library only works as a filter, so building a non-filter version will just build a broken one I think.

[arichardson]

Hmm, I must have been building with -fuse-ld=lld but without setting XLD. Maybe I should add a .error instead?

[jhb]

If you get a build error now that is probably just as well as a .error. I'd be tempted to avoid adding a dedicated .error for an odd edge case if the build already fails.