ceph.spec: build with system libpmem on fedora and el8

ceph.spec: build with system libpmem on fedora and el8

* build with WITH_SYSTEM_PMDK=ON on fedora, as f32 and f33 ship
  libpmem1.8 and libpmem1.9 respectively. and we need libpmem v1.7
* build with WITH_SYSTEM_PMDK=ON on el8, as el8 and CentOS8 AppStream
  ships libpmem v1.6,

quote from nvml.spec:

> By design, PMDK does not support any 32-bit architecture.
> Due to dependency on some inline assembly, PMDK can be compiled only
> on these architectures:
> - x86_64
> - ppc64le (experimental)
> - aarch64 (unmaintained, supporting hardware doesn't exist?)

so far, only x86_64 and ppc64le packages are built.

see also,
https://src.fedoraproject.org/rpms/nvml/blob/rawhide/f/nvml.spec

this change addresses a regression introduced by
a49d1dbb32e2436ff2836a85b2fa84418f0a5fff

Signed-off-by: Kefu Chai <kchai@redhat.com>