link:clarification & enable MachO getGlobalSymbol

This adds clarification to the getGlobalSymbol doc comments,
as well as renames the `addExternFn` function for MachO to `getGlobalSymbol`.
This function will now be called from 'src/link.zig' as well.

Finally, this also enables compiling zig's libc using LLVM even though
the `fno-LLVM` flag is given.

src/Compilation.zig

@@ -1920,8 +1920,7 @@ pub fn create(gpa: Allocator, options: InitOptions) !*Compilation {
         // and make sure the compiler-rt symbols are emitted.
         const capable_of_building_compiler_rt = build_options.have_llvm;
 
-        const capable_of_building_zig_libc = comp.bin_file.options.use_stage1 or
-            comp.bin_file.options.use_llvm;
+        const capable_of_building_zig_libc = build_options.have_llvm;
         const capable_of_building_ssp = comp.bin_file.options.use_stage1;
 
         if (comp.bin_file.options.include_compiler_rt and capable_of_building_compiler_rt) {

src/arch/aarch64/CodeGen.zig

@@ -3188,7 +3188,7 @@ fn airCall(self: *Self, inst: Air.Inst.Index, modifier: std.builtin.CallOptions.
                         lib_name,
                     });
                 }
-                const n_strx = try macho_file.addExternFn(mem.sliceTo(decl_name, 0));
+                const n_strx = try macho_file.getGlobalSymbol(mem.sliceTo(decl_name, 0));
 
                 _ = try self.addInst(.{
                     .tag = .call_extern,

src/arch/x86_64/CodeGen.zig

@@ -3996,7 +3996,7 @@ fn airCall(self: *Self, inst: Air.Inst.Index, modifier: std.builtin.CallOptions.
                         lib_name,
                     });
                 }
-                const n_strx = try macho_file.addExternFn(mem.sliceTo(decl_name, 0));
+                const n_strx = try macho_file.getGlobalSymbol(mem.sliceTo(decl_name, 0));
                 _ = try self.addInst(.{
                     .tag = .call_extern,
                     .ops = undefined,

src/link.zig

@@ -439,14 +439,15 @@ pub const File = struct {
     }
 
     /// Called from within CodeGen to retrieve the symbol index of a global symbol.
-    /// If no symbol exists yet with this name, a new one will be created instead.
+    /// If no symbol exists yet with this name, a new undefined global symbol will
+    /// be created. This symbol may get resolved once all relocatables are (re-)linked.
     pub fn getGlobalSymbol(base: *File, name: []const u8) UpdateDeclError!u32 {
         log.debug("getGlobalSymbol '{s}'", .{name});
         switch (base.tag) {
             // zig fmt: off
             .coff  => unreachable,
             .elf   => unreachable,
-            .macho => unreachable,
+            .macho => return @fieldParentPtr(MachO, "base", base).getGlobalSymbol(name),
             .plan9 => unreachable,
             .spirv => unreachable,
             .c     => unreachable,

src/link/MachO.zig

@@ -5366,7 +5366,7 @@ fn addAtomToSection(self: *MachO, atom: *Atom, match: MatchingSection) !void {
     }
 }
 
-pub fn addExternFn(self: *MachO, name: []const u8) !u32 {
+pub fn getGlobalSymbol(self: *MachO, name: []const u8) !u32 {
     const sym_name = try std.fmt.allocPrint(self.base.allocator, "_{s}", .{name});
     defer self.base.allocator.free(sym_name);
     const n_strx = try self.makeString(sym_name);

src/link/Wasm.zig

@@ -864,7 +864,8 @@ pub fn lowerUnnamedConst(self: *Wasm, tv: TypedValue, decl_index: Module.Decl.In
     return atom.sym_index;
 }
 
-/// Returns the symbol index from the name of an intrinsic.
+/// Returns the symbol index from a symbol of which its flag is set global,
+/// such as an exported or imported symbol.
 /// If the symbol does not yet exist, creates a new one symbol instead
 /// and then returns the index to it.
 pub fn getGlobalSymbol(self: *Wasm, name: []const u8) !u32 {