mk: fix make defconfig on FreeBSD

mk: fix make defconfig on FreeBSD

On FreeBSD, make defconfig generates the config as
"defconfig_x86_64-bsdapp-", which does not resolve to any known
config file.

On FreeBSD, we get amd64 out of "uname -m", which was not handled by
the list of checks, but which now resolves to x86_64-native.

Then we run '$CC --version', and use grep -o with the list of known
compilers, and set to either gcc, icc or clang. Grep's '-o' option
returns the matched word rather than the whole line, making the
result easier to use.

The remaining code in the patch then takes ${compiler}, the "uname -m"
output and assembles them all together into a valid freebsd config name,
i.e. "defconfig_x86_64-native-bsdapp-clang".

Fixes: bce6c42c4ad5 ("mk: add sensible default target with defconfig")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>
Tested-by: Anatoly Burakov <anatoly.burakov@intel.com>