package/bayer2rgb-neon: fix aarch64 build with BR2_OPTIMIZE_0

Fix the following build failure with aarch64 and BR2_OPTIMIZE_0 raised
since commit 0512910e4901edcb78bb1daa4eb3be9bdc4dc8a9:

src/convert-neon-body-bayer8.inc.h: In function 'bayer2rgb_convert_neon':
src/convert-neon.c:233:17: error: third argument to '__builtin_prefetch' must be a constant
  233 |                 __builtin_prefetch(((void const *)(_addr)) + 64, 0, (_lvl)); \
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/convert-neon-body-bayer8.inc.h:8:9: note: in expansion of macro 'do_prefetch'
    8 |         do_prefetch(&in[+1*stride - 1], (rows_per_loop) < 3 ? 3 : 0);
      |         ^~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/307c1c448becd99dfba3a17b3ef816db0bb5cad6

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/bayer2rgb-neon/bayer2rgb-neon.mk

@@ -19,6 +19,14 @@ ifeq ($(BR2_arm),y)
 BAYER2RGB_NEON_CFLAGS += -mfpu=neon
 endif
 
+# __builtin_prefetch() third argument must be a constant, but
+# bayer2rgb-neon uses a variable, derived from a constant, so some
+# optimization is needed to allow the compiler to turn it into a
+# constant, otherwise the build fails
+ifeq ($(BR2_OPTIMIZE_0),y)
+BAYER2RGB_NEON_CFLAGS += -O1
+endif
+
 BAYER2RGB_NEON_CONF_ENV = CFLAGS="$(BAYER2RGB_NEON_CFLAGS)"
 
 $(eval $(autotools-package))