llvm-objcopy: Implement --extract-partition and --extract-main-partition.
This implements the functionality described in
https://lld.llvm.org/Partitions.html. It works as follows:
- Reads the section headers using the ELF header at file offset 0;
- If extracting a loadable partition:
- Finds the section containing the required partition ELF header by looking it up in the section table;
- Reads the ELF and program headers from the section.
- If extracting the main partition:
- Reads the ELF and program headers from file offset 0.
- Filters the section table according to which sections are in the program headers that it read:
- If ParentSegment != nullptr or section is not SHF_ALLOC, then it goes in.
- Sections containing partition ELF headers or program headers are excluded as there are no headers for these in ordinary ELF files.
Differential Revision: https://reviews.llvm.org/D62364
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362818
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