Fix illumos compile error in coreclr/runtime/amd64/AllocFast.S

 on illumos with gcc 13.3 cross copmiler
 coreclr/runtime/amd64/AllocFast.S: Assembler messages:
  coreclr/runtime/amd64/AllocFast.S:237: Error: missing ')'

Author: gwr

src/coreclr/runtime/amd64/AllocFast.S

@@ -233,8 +233,11 @@ LEAF_END RhpNewArrayFast, _TEXT
 LEAF_ENTRY RhpNewPtrArrayFast, _TEXT
 
         // Delegate overflow handling to the generic helper conservatively
+        // The constant 0x8000000 is (0x40000000 / sizeof(void*))
+        // Some assemblers don't like an expression here, so the
+        // constant expression is reduced to it's simple form.
 
-        cmp         rsi, (0x40000000 / 8) // sizeof(void*)
+        cmp         rsi, 0x8000000        // (0x40000000 / 8)
         jae         C_FUNC(RhpNewArrayFast)
 
         // In this case we know the element size is sizeof(void *), or 8 for x64