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

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>