pythongh-111544: Fix mimalloc build on AIX (python#111593)

Author: ayappanec

Objects/mimalloc/prim/unix/prim.c

@@ -50,7 +50,7 @@ terms of the MIT license. A copy of the license can be found in the file
   #include <sys/sysctl.h>
 #endif
 
-#if !defined(__HAIKU__) && !defined(__APPLE__) && !defined(__CYGWIN__)
+#if !defined(__HAIKU__) && !defined(__APPLE__) && !defined(__CYGWIN__) && !defined(_AIX)
   #define MI_HAS_SYSCALL_H
   #include <sys/syscall.h>
 #endif
@@ -76,7 +76,7 @@ static int mi_prim_access(const char *fpath, int mode) {
   return syscall(SYS_access,fpath,mode);
 }
 
-#elif !defined(__APPLE__)  // avoid unused warnings
+#elif !defined(__APPLE__) && !defined(_AIX)  // avoid unused warnings
 
 static int mi_prim_open(const char* fpath, int open_flags) {
   return open(fpath,open_flags);