config/x86: skip GNU binutils bug check for LLVM

AVX512 was disabled when GNU binutils were missing or had a known bug,
even if LLVM binutils were used for the build,
because binutils-avx512-check.sh was invoked regardless and failed.
In particular, this was the case for FreeBSD with clang (default).
Run the check only when GNU binutils are used.

Fixes: 68b1f1cda5b4 ("build: check AVX512 rather than binutils version")
Cc: stable@dpdk.org
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>