[llvm] [llvm-objcopy] Add --gap-fill and --pad-to options (PR #65815)

[James Henderson via llvm-commits]

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+# RUN: yaml2obj %s >%t
+
+# Test gap fill with all allocatable output sections
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jh7370 wrote:

Nit: New llvm-objcopy tests use `##` for the comments, to help them stand out from RUN and CHECK lines. 

Also, this sentence needs a trailing "." - this applies in several other places in the tests too.

This comment doesn't really help me understand the purpose of this test. The test name is `gap-fill-order.test` which implies that it has something to do with the order of something, but it's not clear what that is.

https://github.com/llvm/llvm-project/pull/65815