[libc++][AIX] fix XCOFF magic in symcheck

There are some typos here in how XCOFF magic are defined,
we intended the second two digits to be a hex value, but
our syntax doesn't say that. Thus this will never match.

Author: daltenty

libcxx/utils/libcxx/sym_check/util.py

@@ -95,7 +95,7 @@ def is_xcoff_or_big_ar(filename):
     with open(filename, "rb") as f:
         magic_bytes = f.read(7)
     return (
-        magic_bytes[:4] in [b"\x01DF", b"\x01F7"]  # XCOFF32  # XCOFF64
+        magic_bytes[:2] in [b"\x01\xDF", b"\x01\xF7"]  # XCOFF32  # XCOFF64
         or magic_bytes == b"<bigaf>"
     )