Add comment about patch provenance.

Author: freakboy3742

Include/internal/mimalloc/mimalloc/prim.h

@@ -174,6 +174,8 @@ static inline void* mi_prim_tls_slot(size_t slot) mi_attr_noexcept {
     __asm__ volatile ("mrc p15, 0, %0, c13, c0, 3\nbic %0, %0, #3" : "=r" (tcb));
     res = tcb[slot];
   #elif defined(__aarch64__)
+    // CPython: patched for watchOS support.
+    // Submitted upstream as microsoft/mimalloc#1444
     // Use a 64-bit temporary: on arm64_32 (ILP32), pointers are 32 bits
     // but system registers are 64 bits.
     uint64_t tp; MI_UNUSED(ofs);