eal/ppc: use compiler builtins for atomics

eal/ppc: use compiler builtins for atomics

Replace existing PPC assembly code for rte_atomicXX ops with compiler
atomic builtins as previously adopted by DPDK (see [1] and [2]).  This
has the additional benefit of resolving a POWER10 build failure due to an
outstanding gcc issue which fails on the existing PPC assembly code [3].

[1] https://www.dpdk.org/blog/2021/03/26/dpdk-adopts-the-c11-memory-model/
[2] https://doc.dpdk.org/guides/rel_notes/deprecation.html
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98519

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