[llvm-objcopy] Add --build-id-link-dir flag

[llvm-objcopy] Add --build-id-link-dir flag

This flag does not exist in GNU objcopy but has a major use case.
Debugging tools support the .build-id directory structure to find
debug binaries. There is no easy way to build this structure up
however. One way to do it is by using llvm-readelf and some crazy
shell magic. This implements the feature directly. It is most often
the case that you'll want to strip a file and send the original to
the .build-id directory but if you just want to send a file to the
.build-id directory you can copy to /dev/null instead.

Differential Revision: https://reviews.llvm.org/D54384

llvm-svn: 348174