Improve test_standalone_system_headers. NFC (#22854)

Include `compat` headers.

See #22850

system/include/compat/xlocale.h

@@ -1,6 +1,9 @@
 #ifndef _COMPAT_XLOCALE_H_
 #define _COMPAT_XLOCALE_H_
 
+#define __NEED_locale_t
+#include <bits/alltypes.h>
+
 #include <locale.h>
 
 #ifdef __cplusplus

test/test_other.py

@@ -9349,7 +9349,7 @@ def test_standalone_system_headers(self):
     # everything it needs.
     directories = {'': []}
     for elem in os.listdir(path_from_root('system/include')):
-      if elem in ('compat', 'fakesdl'):
+      if elem == 'fakesdl':
         continue
       full = path_from_root('system/include', elem)
       if os.path.isdir(full):
@@ -9373,19 +9373,22 @@ def test_standalone_system_headers(self):
         cxx_only = header in [
           'wire.h', 'val.h', 'bind.h',
           'webgpu_cpp.h', 'webgpu_cpp_chained_struct.h', 'webgpu_enum_class_bitmasks.h',
+          # Some headers are not yet C compatible
+          'arm_neon.h', 'avxintrin.h', 'immintrin.h',
         ]
-        if directory:
+        if directory and directory != 'compat':
           header = f'{directory}/{header}'
         inc = f'#include <{header}>\n__attribute__((weak)) int foo;\n'
+        cflags = ['-Werror', '-Wall', '-pedantic', '-mavx', '-msimd128', '-msse3']
         if cxx_only:
           create_file('a.cxx', inc)
           create_file('b.cxx', inc)
-          self.run_process([EMXX, '-Werror', '-Wall', '-pedantic', 'a.cxx', 'b.cxx'])
+          self.run_process([EMXX, '-msse3', 'a.cxx', 'b.cxx'] + cflags)
         else:
           create_file('a.c', inc)
           create_file('b.c', inc)
           for std in ([], ['-std=c89']):
-            self.run_process([EMCC] + std + ['-Werror', '-Wall', '-pedantic', 'a.c', 'b.c'])
+            self.run_process([EMCC, 'a.c', 'b.c'] + std + cflags)
 
   @is_slow_test
   @parameterized({