mk: fix clang version query
mk: fix clang version query

-dumpversion is for gcc compatibility and doesn't return actual clang
version. -dumpversion only returns 4.2.1 for a long time.

Fixes: 2ef6eea891e5 ("mk: add clang toolchain")

Signed-off-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>
Tested-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk