eal: cleanup strerror function

eal: cleanup strerror function

When compiled on Ubuntu with extra warnings enabled, the rte_strerror()
function triggered a warning about an unused return value from
strerror_r(). Rather than always have this warning disabled, we fix this,
and in the process do some cleanup of the code so as to reduce the
complexity of the fix, e.g. not having the #ifdef macros inside the
snprintf call.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>