osxbom: switch to lib-style depspec for libunwind

[cooljeanius]

This is another attempt at doing #28635 (except correctly this time); I'm copying and pasting the OP from there, and then editing it to be up-to-date.
In https://trac.macports.org/ticket/66250 it says that Macports-built libunwind can cause problems with other ports. This PR tweaks osxbom's dependency on libunwind to use a lib:-style dependency instead, so that the system libunwind can satisfy it instead. It also removes the unnecessary dependency on libmacho-headers, and reverts the coreutils depspec to use the bin:-style dependency I had originally used, instead of the new path-style dependency (added in d4eea8e) that references a file that this port doesn't even use.

Description

Type(s)

• enhancement

Tested on

macOS 15.5 24F74 x86_64
Xcode 16.4 16F6

Verification

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