[ObjWriter/Mach-O] Mark non-global symbols as .private_extern to match ELF (#106446)

Extracted from #106224

`.private_extern` is the logical equivalent of `.hidden`+`.global` in ELF. We already emit those flags in ELF, so do it in Mach-O too.

Documentation for `.private_extern`:
> It's used to mark symbols with limited visibility. When the file is fed to the static linker, it clears the N_EXT bit for each symbol with the N_PEXT bit set. The ld option -keep_private_externs turns off this behavior.

Author: filipnavara

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/MachNative.cs

@@ -96,6 +96,7 @@ internal static class MachNative
         public const byte N_INDR = 0xA;
         public const byte N_SECT = 0xE;
         public const byte N_PBUD = 0xC;
+        public const byte N_PEXT = 0x10;
 
         // Symbol descriptor flags
         public const ushort REFERENCE_FLAG_UNDEFINED_NON_LAZY = 0;

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/MachObjectWriter.cs

@@ -484,7 +484,7 @@ private protected override void EmitSymbolTable(
                     Section = section,
                     Value = section.VirtualAddress + (ulong)definition.Value,
                     Descriptor = N_NO_DEAD_STRIP,
-                    Type = N_SECT | N_EXT,
+                    Type = (byte)(N_SECT | N_EXT | (definition.Global ? 0 : N_PEXT)),
                 });
             }
             sortedDefinedSymbols.Sort((symA, symB) => string.CompareOrdinal(symA.Name, symB.Name));