[llvm-objcopy] Use SHT_NOTE for added note sections.

Summary:
Fix llvm-objcopy to add .note sections as SHT_NOTEs. GNU objcopy overrides section flags for special sections. For `.note` sections (with the exception of `.note.GNU-stack`), SHT_NOTE is used.

Many other sections are special cased by libbfd, but `.note` is the only special section I can seem to find being used with objcopy --add-section.

See `.note` in context of the full list of special sections here: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/elf.c;h=eb3e1828e9c651678b95a1dcbc3b124783d1d2be;hb=HEAD#l2675

Reviewers: jhenderson, alexshap, jakehehrlich, espindola

Reviewed By: jhenderson

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D56570

llvm-svn: 351204