eliminate dependency on LLD for Mach-O

[andrewrk]

This is a sub-task of #8726.

LLD on Mach-O is fatally flawed for a few reasons. One example is #3295.

However Zig still falls back to LLD for Mach-O in a handful of cases. Here is the current logic:

zig/src/Compilation.zig

Lines 714 to 751 in 2d11967

	// Make a decision on whether to use LLD or our own linker.
	const use_lld = if (options.use_lld) |explicit| explicit else blk: {
	if (!build_options.have_llvm)
	break :blk false;
	if (ofmt == .c)
	break :blk false;
	if (options.want_lto) |lto| {
	if (lto) {
	break :blk true;
	}
	}
	// Our linker can't handle objects or most advanced options yet.
	if (options.link_objects.len != 0 or
	options.c_source_files.len != 0 or
	options.frameworks.len != 0 or
	options.system_libs.len != 0 or
	options.link_libc or options.link_libcpp or
	options.link_eh_frame_hdr or
	options.link_emit_relocs or
	options.output_mode == .Lib or
	options.lld_argv.len != 0 or
	options.image_base_override != null or
	options.linker_script != null or options.version_script != null)
	{
	break :blk true;
	}
	if (use_llvm) {
	// If stage1 generates an object file, self-hosted linker is not
	// yet sophisticated enough to handle that.
	break :blk options.root_pkg != null;
	}
	break :blk false;
	};

zig/src/link/MachO.zig

Lines 637 to 656 in 2d11967

	const use_zld = blk: {
	if (self.base.options.is_native_os and self.base.options.system_linker_hack) {
	// If the user forces the use of ld64, make sure we are running native!
	break :blk false;
	}
	if (self.base.options.target.cpu.arch == .aarch64) {
	// On aarch64, always use zld.
	break :blk true;
	}
	if (self.base.options.output_mode == .Lib or
	self.base.options.linker_script != null)
	{
	// Fallback to LLD in this handful of cases on x86_64 only.
	break :blk false;
	}
	break :blk true;
	};

This issue is to improve self-hosted linking on Mach-O enough that for Mach-O targets, we always use self-hosted linking, and we can remove this line from cmake:

zig/cmake/Findlld.cmake

Line 45 in 2d11967

FIND_AND_ADD_LLD_LIB(lldMachO)