xtensa: fix CODA build

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>
diff --git a/arch/xtensa/Makefile b/arch/xtensa/Makefile
index f973754..b201d44 100644
--- a/arch/xtensa/Makefile
+++ b/arch/xtensa/Makefile
@@ -31,7 +31,7 @@
 export PLATFORM
 
 # temporarily until string.h is fixed
-KBUILD_CFLAGS += -ffreestanding
+KBUILD_CFLAGS += -ffreestanding -D__linux__
 
 KBUILD_CFLAGS += -pipe -mlongcalls