gcc/gcc-final: Disable libcilkrts when using static libs

Disable libcilkrts when building static, as there is no static version:
https://software.intel.com/en-us/articles/intel-cilk-plus-runtime-library-libcilkrts-can-only-be-linked-dynamically/

Fixes the following toolchain build error when building for i386 and
BR2_STATIC_LIBS=y + BR2_TOOLCHAIN_BUILDROOT_CXX=y is set:

../../../libcilkrts/runtime/sysdep-unix.c:603:19:
fatal error: dlfcn.h: No such file or directory

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/gcc/gcc-final/gcc-final.mk

@@ -79,8 +79,9 @@ HOST_GCC_FINAL_CONF_OPTS += --disable-symvers
 endif
 
 # Disable shared libs like libstdc++ if we do static since it confuses linking
+# In that case also disable libcilkrts as there is no static version
 ifeq ($(BR2_STATIC_LIBS),y)
-HOST_GCC_FINAL_CONF_OPTS += --disable-shared
+HOST_GCC_FINAL_CONF_OPTS += --disable-shared --disable-libcilkrts
 else
 HOST_GCC_FINAL_CONF_OPTS += --enable-shared
 endif