[llvm-objcopy] Add support for --strip-sections to remove all section headers...

[llvm-objcopy] Add support for --strip-sections to remove all section headers leaving only program headers and loadable segment data

elf utils implements a particularly extreme form of stripping that I'd
like to support. eu-strip has an option called "strip-sections" that
removes all section headers and leaves only program headers and the
segment data. I have implemented this option partly as a test but mainly
because in Fuchsia we would like to use this option to minimize the size
of our executables. The other strip options that are on my list include
--strip-all and --strip-debug. This is a preliminary implementation that
I'd like to start using in Fuchsia builds if possible. This change
implements such a stripping option for llvm-objcopy

Differential Revision: https://reviews.llvm.org/D38335

llvm-svn: 315412