pmem: fix BUG() error in pmem.h:48 on X86_32

After 'commit fc0c2028

 ("x86, pmem: use memcpy_mcsafe()
for memcpy_from_pmem()")', probing a PMEM device hits the BUG()
error below on X86_32 kernel.

 kernel BUG at include/linux/pmem.h:48!

memcpy_from_pmem() calls arch_memcpy_from_pmem(), which is
unimplemented since CONFIG_ARCH_HAS_PMEM_API is undefined on
X86_32.

Fix the BUG() error by adding default_memcpy_from_pmem().

Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>