[libc++][AIX] Fixup problems with ABI list checking (#155643)

There are some problems with our ABI list checking exposed by recent
compiler/cmake upgrades.

- For symcheck, there are typos 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 a valid XCOFF file.
- AIX triples can have version numbers. Those need to be discarded when
looking for an libc++ ABI list, like we do for other targets.

Author: daltenty

libcxx/lib/abi/CMakeLists.txt

@@ -16,6 +16,9 @@ function(cxx_abi_list_identifier result triple abi_library abi_version unstable
   elseif("${triple}" MATCHES "freebsd")
     # Ignore the major and minor versions of freebsd targets.
     string(REGEX REPLACE "freebsd[0-9]+\\.[0-9]+" "freebsd" triple "${triple}")
+  elseif("${triple}" MATCHES "aix")
+    # Ignore the V.R.M.F version string of aix targets.
+    string(REGEX REPLACE "aix[0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+" "aix" triple "${triple}")
   endif()
   list(APPEND abi_properties "${triple}")
   list(APPEND abi_properties "${abi_library}")

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>"
     )