[libcxx] [test] Fix the locale ctype widen tests on AIX (#91744)

The C locale on AIX uses `ISO-8859-1`, where `0xFB` is a valid
character. Widening char(-5) succeeds and produces L'\u00fb' the same as
on macOS, FreeBSD, and Windows. This patch removes `XFAIL:
LIBCXX-AIX-FIXME` and uses the macOS, FreeBSD, and WIN32 code path for
AIX.

Author: xingxue-ibm

libcxx/test/std/localization/locale.categories/category.ctype/locale.ctype.byname/widen_1.pass.cpp

@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: locale.en_US.UTF-8
-// XFAIL: LIBCXX-AIX-FIXME
 // XFAIL: no-wide-characters
 
 // <locale>
@@ -57,7 +56,7 @@ int main(int, char**)
             assert(f.widen('.') == L'.');
             assert(f.widen('a') == L'a');
             assert(f.widen('1') == L'1');
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(_WIN32)
+#if defined(__APPLE__) || defined(__FreeBSD__) || defined(_WIN32) || defined(_AIX)
             assert(f.widen(char(-5)) == L'\u00fb');
 #else
             assert(f.widen(char(-5)) == wchar_t(-1));

libcxx/test/std/localization/locale.categories/category.ctype/locale.ctype.byname/widen_many.pass.cpp

@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: locale.en_US.UTF-8
-// XFAIL: LIBCXX-AIX-FIXME
 // XFAIL: no-wide-characters
 
 // <locale>
@@ -63,7 +62,7 @@ int main(int, char**)
             assert(v[3] == L'.');
             assert(v[4] == L'a');
             assert(v[5] == L'1');
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(_WIN32)
+#if defined(__APPLE__) || defined(__FreeBSD__) || defined(_WIN32) || defined(_AIX)
             assert(v[6] == L'\xfb');
 #else
             assert(v[6] == wchar_t(-1));