libmpeg2: fix PPC regression

00d1f50e introduced a regression for PPC
since the `signal()` based detection for altivec in libmpeg2 is not thread
safe unlike our own. here we restore use of our own detection for PPC,
bypassing libmpeg2's unsafe auto-detection otherwise triggered in
`mpeg2_init()`.

fixes #25948.

modules/codec/libmpeg2.c

@@ -197,6 +197,11 @@ static int OpenDecoder( vlc_object_t *p_this )
     p_sys->p_gop_user_data = NULL;
     p_sys->i_gop_user_data = 0;
 
+#if defined( __powerpc__ ) || defined( __ppc__ ) || defined( __ppc64__ )
+    /* libmpeg's detection is not thread safe for ppc-altivec */
+    mpeg2_accel( vlc_CPU_ALTIVEC() ? MPEG2_ACCEL_PPC_ALTIVEC : 0 );
+#endif
+
     /* Initialize decoder */
     p_sys->p_mpeg2dec = mpeg2_init();
     if( p_sys->p_mpeg2dec == NULL)