pmdinfogen: allow using stdin and stdout

Rather than having to work off files all the time, allow stdin and stdout
to be used as the source and destination for pmdinfogen. This will allow
other possible usages from scripts, e.g. taking files from ar archive and
building a single .pmd.c file from all the .o files in it.

	for f in `ar t librte_pmd_xyz.a` ; do
		ar p librte_pmd_xyz.a $f | pmdinfogen - - >> xyz_info.c
	done

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>