arm: Use -masm-syntax-unified when compiling with gcc8 or newer

Annoyingly, this makes all of the '.S' files we compile get treated as
divided syntax, so we need to make the syntax in them explicit.

Change-Id: I56a3916b7b24c84a1214a5d6bc4ed4d651f002cf

lib/rbcodec/codecs/demac/libdemac/udiv32_arm.S

@@ -29,6 +29,9 @@
  ****************************************************************************/
 
 #include "config.h"
+
+.syntax unified
+
 /* On targets with codec iram, a header file will be generated after an initial
    link of the APE codec, stating the amount of IRAM remaining for use by the
    reciprocal lookup table. */

lib/rbcodec/codecs/libtta/filter_arm.S

@@ -26,6 +26,8 @@
  * void hybrid_filter(fltst *fs, int *in)
  */
 
+    .syntax unified
+
 #ifdef USE_IRAM
     .section .icode, "ax", %progbits
 #else