Move toolchain utils to -toolchain package

[ivy]

Summary

Move LLVM ar, nm, and size. Also move elftoolchain strings, which
should have already been there.

The remaining LLVM utilities (including strings) stay in -clang for now
since they're links to other executables.

[dim]

LGTM

[emaste]

LGTM.

strings(1) has always been a bit special -- we included it even under WITHOUT_TOOLCHAIN, so not having it in the toolchain pkg made some sense. But I'm fine with it either way.