package/opus: force arm mode instead of Thumb mode

Fix the following build failure:

/tmp/cckXvlOO.s:352: Error: selected processor does not support `smull r6,ip,r5,r0' in Thumb mode

Fixes:
 - http://autobuild.buildroot.org/results/fdedcc8f68651ec34417d609b40effbac456c80d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: tweak comment]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/opus/opus.mk

@@ -12,6 +12,13 @@ OPUS_INSTALL_STAGING = YES
 
 OPUS_CFLAGS = $(TARGET_CFLAGS)
 
+# opus has ARM assembly optimizations not compatible with thumb1:
+# Error: selected processor does not support `smull r6,ip,r5,r0' in Thumb mode
+# so force ARM mode
+ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
+OPUS_CFLAGS += -marm
+endif
+
 ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_85180),y)
 OPUS_CFLAGS += -O0
 endif