[test][objcopy] Replace GNU sed extension with BSD compatible syntax.

GNU sed offers the `,+4d` to delete the line a next four lines, but BSD
sed doesn't seem to support it (at least in macOS 10.15, but seems to do
in my 11.6 version).

Replace the usage of the extension with the equivalent syntax that works
both in BSD and GNU sed. I don't have a macOS 10.15 to check, but this
works in both my macOS 11.6 and Linux machines.

Differential Revision: https://reviews.llvm.org/D112583