[libcxx] Work around picolibc argv handling in tests. (#127662)

This fixes some test failures when the libcxx tests are run against an
up-to-date picolibc on embedded Arm, because those tests depend on an
unsupported locale but the `hasAnyLocale` preliminary check wrongly
concluded that the locale _was_ supported.

`hasAnyLocale` passes a set of locale strings to a test program via the
command line, and checks if the libc under test reports that any of the
locales can be successfully set via setlocale(). In some invocations one
of the locale names contains a space, e.g. the Windows-style locale name
"English_United States.1252".

Unfortunately picolibc's crt0, when running under Arm semihosting,
fetches the single command string from the host and then splits it up at
spaces without implementing any kind of quoting. So it simply isn't
possible to get a space into an argv word. As a result, we end up
testing for the locale (in this example) "English_United". In up-to-date
versions of picolibc, this is actually accepted, since it contains no
objectionable character set specification (or indeed any at all). So the
lit check wrongly concludes that libc supports that locale, and enables
some locale tests, which fail.

This patch works around the issue entirely within `hasAnyLocale()`, by
abandoning the use of argv completely, and instead encoding the list of
locales to check as an array of strings inside the test program.

Author: statham-arm

libcxx/utils/libcxx/test/dsl.py

@@ -9,7 +9,6 @@
 import os
 import pickle
 import platform
-import shlex
 import shutil
 import tempfile
 
@@ -274,23 +273,41 @@ def hasAnyLocale(config, locales):
     %{exec} -- this means that the command may be executed on a remote host
     depending on the %{exec} substitution.
     """
-    program = """
+
+    # Convert the locale names into C string literals. We expect all currently
+    # known locale names to be printable ASCII and not contain awkward
+    # characters like \ or ", so this should be trivial.
+    assert all(
+        0x20 <= ord(ch) <= 0x7E and ch not in {'"', "\\"}
+        for locale in locales
+        for ch in locale
+    )
+    name_string_literals = ", ".join('"' + locale + '"' for locale in locales)
+
+    program = (
+        """
     #include <stddef.h>
     #if defined(_LIBCPP_VERSION) && !_LIBCPP_HAS_LOCALIZATION
       int main(int, char**) { return 1; }
     #else
       #include <locale.h>
-      int main(int argc, char** argv) {
-        for (int i = 1; i < argc; i++) {
-          if (::setlocale(LC_ALL, argv[i]) != NULL) {
+      static const char *const test_locale_names[] = {
+          """
+        + name_string_literals
+        + """, nullptr,
+      };
+      int main() {
+        for (size_t i = 0; test_locale_names[i]; i++) {
+          if (::setlocale(LC_ALL, test_locale_names[i]) != NULL) {
             return 0;
           }
         }
         return 1;
       }
     #endif
-  """
-    return programSucceeds(config, program, args=[shlex.quote(l) for l in locales])
+    """
+    )
+    return programSucceeds(config, program)
 
 
 @_memoizeExpensiveOperation(lambda c, flags="": (c.substitutions, c.environment, flags))