This boilerplate should be handled in CopyConfig instead of copied in every platform-specific implementation, can you add a FIXME to consolidate it?
SecName is actually the pair of <segment>,<section>, so shouldn't SecName be replaced with Pair.second? I think the test case you have shows this.
If the segment can't be created, it seems like it's not at all related to the file content, so either the filename here should be the file we're trying to add a segment to, or it should be a regular StringError.
Is there a utility you could extract to convert from possibly-null-delimited segment/section names to StringRefs? This pattern is used above with the strncpy and also in MachOReader
Don't think I have any comments besides these two, so LGTM once they're addressed & you get a MachO reviewer to be happy w/ this.
sizeof(Sec.sectname) should be sizeof(Sec.segname) here?
A common use of GNU objcopy is to run with --add-section .note.GNU-stack=/dev/null, which I imagine corresponds to a 0-sized section. I think you might want to wrap OwnedContentData in Optional<>. (Assuming a zero-sized section is valid in Mach-O). Additionally, can you add a test for --add-section w/ /dev/null?
The ELF code doesn't use Optional because it takes a different approach by having subclasses for different types of sections, so OwnedDataSection is different from Section -- I don't know if that makes sense here
There seems to be some stuff to do with 64-bit versus normal segments in here, which I don't fully follow due to it being Mach-O, but your test only seems to handle the 64-bit segment type. Should it be extended?
It probably makes sense to call %t %t.64bit.
By the way, there seems to be a degree of consensus that -o should be preferred to a shell redirection with yaml2obj. No need to modify old tests, but if you're adding new ones/updating them, it's probably good to make this change too.
%t.out1 -> %t.out1.64bit or something along those lines
This and the above block are very similar. You might want to consider pulling them into some kind of function to reduce the duplication.