macho: use start.zig for macOS entrypoint

This effectively allows us to compile

```zig
pub fn main() void {}
```

which then calls into `std.start`.

Changes required to make this happen:
* handle signed int to immediate in x86_64 and aarch64 codegen
* ensure that on arm64 macOS, `.x19` is a caller-preserved register -
  I'm not sure about that one at all and would like to brainstorm it
  with anyone interested and especially Joachim.
* finally, fix a bug in the linker - mark new got entry as dirty upon
  atom growth.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/arch/aarch64/CodeGen.zig b/src/arch/aarch64/CodeGen.zig
index a763651825..d3c72fe1b1 100644
--- a/src/arch/aarch64/CodeGen.zig
+++ b/src/arch/aarch64/CodeGen.zig
@@ -2393,6 +2393,9 @@ fn genTypedValue(self: *Self, typed_value: TypedValue) InnerError!MCValue {
         },
         .Int => {
             const info = typed_value.ty.intInfo(self.target.*);
+            if (info.bits <= ptr_bits and info.signedness == .signed) {
+                return MCValue{ .immediate = @bitCast(u64, typed_value.val.toSignedInt()) };
+            }
             if (info.bits > ptr_bits or info.signedness == .signed) {
                 return self.fail("TODO const int bigger than ptr and signed int", .{});
             }