[llvm-objcopy] Implement --set-section-flags.

Summary:
--set-section-flags is used to change the section flags (e.g. SHF_ALLOC) for given sections. The flags allowed are the same from the existing --rename-section=.old=.new[,flags] feature.

Additionally, make sure that --set-section-flag cannot be used with --rename-section (either the source or destination), since --rename-section accepts flags. This avoids ambiguity for something like "--rename-section=.foo=.bar,alloc --set-section-flag=.bar,code".

Reviewers: jhenderson, jakehehrlich, alexshap, espindola

Reviewed By: jhenderson, jakehehrlich

Subscribers: llvm-commits, emaste, arichardson

Differential Revision: https://reviews.llvm.org/D57198

llvm-svn: 352505