llvm: Also apply the nobuiltin attribute for the no_builtin module option.

From `zig build-exe --help`:

  -fno-builtin              Disable implicit builtin knowledge of functions

It seems entirely reasonable and even expected that this option should imply
both no-builtins on functions (which disables transformation of recognized code
patterns to libcalls) and nobuiltin on call sites (which disables transformation
of libcalls to intrinsics). We now match Clang's behavior for -fno-builtin.

In both cases, we're painting with a fairly broad brush by applying this to an
entire module, but it's better than nothing. ziglang#21833 proposes a more fine-grained
way to apply nobuiltin.

Author: alexrp

src/codegen/llvm.zig

@@ -5576,6 +5576,10 @@ pub const FuncGen = struct {
         var attributes: Builder.FunctionAttributes.Wip = .{};
         defer attributes.deinit(&o.builder);
 
+        if (self.ng.ownerModule().no_builtin) {
+            try attributes.addFnAttr(.nobuiltin, &o.builder);
+        }
+
         switch (modifier) {
             .auto, .never_tail, .always_tail => {},
             .never_inline => try attributes.addFnAttr(.@"noinline", &o.builder),