[llvm-objcopy] Add --strip-debug

Many projects use this option. There are two ways to use it. You can
either a) Just use --strip-debug and keep the old file with debug
content or b) you can use --strip-debug, --only-keep-debug, and
--add-gnu-debuglink all in conjunction to create two separate files, the
stripped file and the debug file. --only-keep-debug is more complicated
than --strip-debug because it keeps the section headers without keeping
section contents. That's not really supported by llvm-objcopy at the
moment but I plan on adding it. So this change just supports a) and
options to support b) will come soon.

Differential Revision: https://reviews.llvm.org/D39919

llvm-svn: 318094