macho: implement pruning of unused segments and sections This is a prelude to a more elaborate work which will implement `-dead_strip` flag - garbage collection of unreachable atoms. Here, when sorting sections, we also check that the section is actually populated with some atoms, and if not, we exclude it from the final linked image. This can happen when we do not import any symbols from dynamic libraries in which case we will not be populating the stubs sections or the GOT table, implying we can skip allocating those sections. Furthermore, we also make a check that a segment is actually occupied too, with the exception of `__TEXT` segment which is non-optional given that it wraps the header and load commands and thus is required by the `dyld` to perform dynamic linking, and `__PAGEZERO` which is generally non-optional when the linked image is an executable. For any other segment, if its section count is zero, we mark it as dead and skip allocating it and generating a load command for it. This commit also includes some minor improvements to the linker such as refactoring of the segment allocating codepaths, skipping `__PAGEZERO` generation for dylibs, and skipping generation of zero-sized atoms for special symbols such as `__mh_execute_header` and `___dso_handle`. These special symbols are only allocated local and global symbol pair and their VM addresses is set to the start of the `__TEXT` segment, but no `Atom` is created, as it's not necessary given that they never carry any machine code. Finally, we now always force-link against `libSystem` which turns out to be required for `dyld` to properly handle `LC_MAIN` load command on older macOS versions such as 10.15.7.