[llvm-objcopy] Implement support for section groups

[llvm-objcopy] Implement support for section groups

This diff adds support for SHT_GROUP sections to llvm-objcopy.
Some sections are interrelated and comprise a group.
For example, a definition of an inline function might require,
in addition to the section containing its instructions,
a read-only data section containing literals referenced inside the function.
A section of the type SHT_GROUP contains the indices of the group members,
therefore, it needs to be updated whenever the indices change.
Similarly, the fields sh_link, sh_info should be recalculated as well.

[Resubmit r328012 with the proper handling of endianness]

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D43996

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328143 91177308-0d34-0410-b5e6-96231b3b80d8