missing_syscall: memfd_create() is supported by glibc since 2.27

Now, our baseline of glibc is 2.31, hence it is OK to drop it.

src/basic/missing_mman.h

@@ -5,24 +5,14 @@
 
 #include "macro.h"
 
-#ifndef MFD_ALLOW_SEALING
-#  define MFD_ALLOW_SEALING 0x0002U
-#else
-assert_cc(MFD_ALLOW_SEALING == 0x0002U);
-#endif
-
-#ifndef MFD_CLOEXEC
-#  define MFD_CLOEXEC 0x0001U
-#else
-assert_cc(MFD_CLOEXEC == 0x0001U);
-#endif
-
+/* since glibc-2.38 */
 #ifndef MFD_NOEXEC_SEAL
 #  define MFD_NOEXEC_SEAL 0x0008U
 #else
 assert_cc(MFD_NOEXEC_SEAL == 0x0008U);
 #endif
 
+/* since glibc-2.38 */
 #ifndef MFD_EXEC
 #  define MFD_EXEC 0x0010U
 #else