MachO linker now handles `-needed-l<name>`, `-needed_library=<name>`
and `-needed_framework=<name>`. While on macOS `-l` is equivalent
to `-needed-l`, and `-framework` to `-needed_framework`, it can be
used to the same effect as on Linux if combined with `-dead_strip_dylibs`.
This commit also adds handling for `-needed_library` which is macOS
specific flag only (in addition to `-needed-l`).
Finally, in order to leverage new linker testing harness, this commit
added ability to specify lowering to those flags via `build.zig`:
`linkSystemLibraryNeeded` (and related), and `linkFrameworkNeeded`.
This approach is more inline with what LLVM/LLD does for testing
of their output, and seems to be more generic and easier to extend
than implementing a lot of repetitive and nontrivial comparison
logic when working directly on structures.
CheckMachOStep specialises CheckFileStep into directed (surgical)
MachO file fuzzy searches. This will be the building block for
comprehensive MachO linker tests.
