contrib: ffmpeg: Use -mincoming-stack-boundary=4

The issue here is that android-x86 doesn't guarantee an aligned stack, which means some of the inline asm functions will waste an extra register on realigning the stack.

libavcodec does however have stuff in place in all entrypoints that realign the stack on all calls into the library. If you build with --extra-cflags='-mincoming-stack-boundary=4', gcc assumes that the stack actually is aligned already, which the entry points ensure, so this inline asm builds fine.
Pointed-out-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Rafaël Carré <funman@videolan.org>

contrib/src/ffmpeg/rules.mak

@@ -87,6 +87,17 @@ endif
 # Linux
 ifdef HAVE_LINUX
 FFMPEGCONF += --target-os=linux --enable-pic
+
+ifeq ($(ANDROID_ABI), x86)
+ifdef HAVE_ANDROID
+# Android-x86 gcc doesn't guarantee an aligned stack, but this is
+# handled by __attribute__((force_align_arg_pointer)) in libavcodec
+# already, so we tell gcc to assume this alignment, so we don't need
+# to waste a precious register in assembly functions to realign it.
+FFMPEG_CFLAGS += -mincoming-stack-boundary=4
+endif # HAVE_ANDROID
+endif # x86
+
 endif
 
 # Windows