config: fix build on RHEL 7.6 for Power9
config: fix build on RHEL 7.6 for Power9

gcc 4.8.5 used on RHEL 7.6 can identify a Power 9 CPU but cannot generate
Power 9 code when the "-mcpu=native" command line argument is used. Test
whether the compiler can generate Power 9 code and adjust the machine
setting appropriately.

Signed-off-by: David Christensen <9a876b0e626a7e524926f129e2e0ea9c4c104ff4@linux.vnet.ibm.com>

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