Suppress armcc warnings about unknown attributes

Originally committed as revision 21950 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2742,6 +2742,7 @@ elif enabled clang; then
 elif enabled armcc; then
     # 2523: use of inline assembler is deprecated
     add_cflags -Wrvct,--diag_suppress=2523
+    add_cflags -Wrvct,--diag_suppress=1207
 elif enabled tms470; then
     add_cflags -pds=824 -pds=837
 fi