toolchain: gcc: fix GCC < 11.0.0 build with -Wundef

When building using GCC version < 11.0.0, TOOLCHAIN_CLANG_VERSION is not
yet defined in gcc.h. Nevertheless, it's still needed due to reuse of
gcc.h in llvm.h.

Guard access to TOOLCHAIN_CLANG_VERSION to fix that.

Signed-off-by: Igor Druzhinin <igord@xilinx.com>