eal/ppc: fix build with gcc 9.3

eal/ppc: fix build with gcc 9.3

Building DPDK on Ubuntu 20.04 with GCC 9.3.0 results in a "subscript is
outside array bounds" message in rte_memcpy function.  The build error
is caused by an interaction between __builtin_constant_p and
"-Werror=array-bounds" as described in this bugzilla:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90387

Modify the code to disable the array-bounds check for GCC versions 9.0
to 9.3.

Cc: stable@dpdk.org
Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>