Use the platform of the source Mach-O file unless ignore_platform

[leptos-null]

On base branch:

$ bin/tbd -p --replace-archs armv7 armv7s arm64 -v v3 /usr/lib/ssh-keychain.dylib | head
--- !tapi-tbd-v3
archs:                 [ armv7, armv7s, arm64 ]
platform:              (null)
flags:                 [ flat_namespace ]
install-name:          "/usr/lib/ssh-keychain.dylib"
current-version:       1
compatibility-version: 1
objc-constraint:       retain_release
exports:
  - archs:                [ armv7, armv7s, arm64 ]

Note that platform is null.

With PR:

$ bin/tbd -p --replace-archs armv7 armv7s arm64 -v v3 /usr/lib/ssh-keychain.dylib | head
--- !tapi-tbd-v3
archs:                 [ armv7, armv7s, arm64 ]
platform:              macosx
flags:                 [ flat_namespace ]
install-name:          "/usr/lib/ssh-keychain.dylib"
current-version:       1
compatibility-version: 1
objc-constraint:       retain_release
exports:
  - archs:                [ armv7, armv7s, arm64 ]

---

I think that the tbd platform should be selected from the source Mach-O in all cases except if ignore_platform is it. ignore_targets implies ignore_platform.
In the handle_targets_platform_and_uuid function that this PR changes, ignore_targets is checked first, so I believe both of these cases are handled.