Build arm32 libc++ for the NDK with -mthumb

Bug: android/ndk#1953

Change-Id: I6d51c995450e98490a70d0ac745bb46be601b102
Test: build.py and check size of arm32 libc++
(cherry picked from commit 44302de)

builders.py

@@ -1007,6 +1007,13 @@ def output_dir(self) -> Path:
         suffix = '-apex' if self._config.extra_config['apex'] else ''
         return old_path.parent / (old_path.name + suffix)
 
+    @property
+    def cflags(self) -> list[str]:
+        result = super().cflags
+        if self._config.target_arch is hosts.Arch.ARM and self._is_ndk:
+            result.append('-mthumb')
+        return result
+
     @property
     def cxxflags(self) -> list[str]:
         base = super().cxxflags