libpam: fix 64-bit pam plugins not installed when add to 32-bit image

While the BSP is configured as a 64-bit kernel and 32-bit userspace,
add a 64-bit version of libpam to the filesystem, there was a failure:
...
| Computing transaction...error: Can't install
pam-plugin-unix-1.1.6-r2@lib64_x86_64: no package provides libpam-lib64
|
| Saving cache...
...

While using 'lib64' as the multilib suffix of libpam RPROVIDES , the
RPROVIDES was overridden by map_depends_variable in classextend.py.
...
 $RPROVIDES_lib64-libpam [2 operations]
   set data_smart.py:429 [finalize]
     " libpam-${baselib}"
   set classextend.py:71 [map_depends_variable]4532
     "lib64-libpam"
 computed:
   "lib64-libpam"
RPROVIDES_lib64-libpam="lib64-libpam"
...

Rename the suffix could fix this issue.
[YOCTO #4532]

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>