Add -mllvm -stack-alignment=16 to CFLAGS when using clang

Originally committed as revision 25027 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2877,6 +2877,7 @@ elif enabled gcc; then
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes
 elif enabled clang; then
+    check_cflags -mllvm -stack-alignment=16
     check_cflags -Qunused-arguments
 elif enabled armcc; then
     # 2523: use of inline assembler is deprecated