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.

4 files changed, 20 insertions, 3 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", .{});
             }
diff --git a/src/arch/aarch64/bits.zig b/src/arch/aarch64/bits.zig
index dfda04da85..f751170818 100644
--- a/src/arch/aarch64/bits.zig
+++ b/src/arch/aarch64/bits.zig
@@ -1,4 +1,5 @@
 const std = @import("std");
+const builtin = @import("builtin");
 const DW = std.dwarf;
 const assert = std.debug.assert;
 const testing = std.testing;
@@ -58,10 +59,19 @@ pub const Register = enum(u6) {
 
 // zig fmt: on
 
-pub const callee_preserved_regs = [_]Register{
-    .x19, .x20, .x21, .x22, .x23,
-    .x24, .x25, .x26, .x27, .x28,
+const callee_preserved_regs_impl = if (builtin.os.tag.isDarwin()) struct {
+    pub const callee_preserved_regs = [_]Register{
+        .x20, .x21, .x22, .x23,
+        .x24, .x25, .x26, .x27,
+        .x28,
+    };
+} else struct {
+    pub const callee_preserved_regs = [_]Register{
+        .x19, .x20, .x21, .x22, .x23,
+        .x24, .x25, .x26, .x27, .x28,
+    };
 };
+pub const callee_preserved_regs = callee_preserved_regs_impl.callee_preserved_regs;
 
 pub const c_abi_int_param_regs = [_]Register{ .x0, .x1, .x2, .x3, .x4, .x5, .x6, .x7 };
 pub const c_abi_int_return_regs = [_]Register{ .x0, .x1, .x2, .x3, .x4, .x5, .x6, .x7 };
diff --git a/src/arch/x86_64/CodeGen.zig b/src/arch/x86_64/CodeGen.zig
index b590937110..d0405c5a00 100644
--- a/src/arch/x86_64/CodeGen.zig
+++ b/src/arch/x86_64/CodeGen.zig
@@ -3178,6 +3178,9 @@ 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", .{});
             }
diff --git a/src/link/MachO.zig b/src/link/MachO.zig
index 9c3dc36def..d4ddf80094 100644
--- a/src/link/MachO.zig
+++ b/src/link/MachO.zig
@@ -3285,6 +3285,7 @@ fn placeDecl(self: *MachO, decl: *Module.Decl, code_len: usize) !*macho.nlist_64
                     .seg = self.data_const_segment_cmd_index.?,
                     .sect = self.got_section_index.?,
                 }).? + 1);
+                got_atom.dirty = true;
             }
 
             symbol.n_value = vaddr;