mach_o: support two level namespace

Closes https://github.com/dart-lang/sdk/pull/63116

GitOrigin-RevId: 8eb01fcda0586e02ec659285b357412e044b823c
Change-Id: I85a2baaa046d207fe1b928043e1e479d3a74a3d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/493000
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

runtime/platform/mach_o.h

@@ -103,6 +103,8 @@ static constexpr uint32_t MH_DYLDLINK = 0x4;
 // The object file does not re-export any of its input dynamic
 // libraries.
 static constexpr uint32_t MH_NO_REEXPORTED_DYLIBS = 0x100000;
+// The object file uses two-level namespace bindings.
+static constexpr uint32_t MH_TWOLEVEL = 0x80;
 
 struct load_command {
   // The tag that specifies the load command for the following

runtime/vm/mach_o.cc

@@ -1932,7 +1932,7 @@ class MachOHeader : public MachOContents {
   uint32_t flags() const {
     if (type_ == SnapshotType::Snapshot) {
       return mach_o::MH_NOUNDEFS | mach_o::MH_DYLDLINK |
-             mach_o::MH_NO_REEXPORTED_DYLIBS;
+             mach_o::MH_NO_REEXPORTED_DYLIBS | mach_o::MH_TWOLEVEL;
     }
     ASSERT(type_ == SnapshotType::DebugInfo || type_ == SnapshotType::Object);
     return 0;