pmix: issue finding libpmix.so when installed in lib64

[abrooks98]

When using --with-pmix=<path> to use pmix as the process manager, configure only searches in <path>/lib and does not also search <path>/lib64. Example on Aurora:

> ../configure --with-ofi=/opt/cray/libfabric/1.22.0 --disable-fortran --with-pmix=/usr --enable-ofi-mr=basic --disable-ofi-inject --enable-ofi-hmem --disable-bounce-buffers --enable-ofi-manual-progress --enable-mr-endpoint --disable-nonfetch-amo --enable-manual-progress
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
...
checking if user requested external PMIx support(/usr)... yes
checking --with-external-pmix value... sanity check ok (/usr/include)
checking --with-external-libpmix value... not found
configure: WARNING: Expected file /usr/lib/libpmix.* not found
configure: error: Cannot continue
> ls /usr/lib/libpmix.*
ls: cannot access '/usr/lib/libpmix.*': No such file or directory
> ls /usr/lib64/libpmix.*
/usr/lib64/libpmix.la  /usr/lib64/libpmix.so  /usr/lib64/libpmix.so.2  /usr/lib64/libpmix.so.2.9.5

Relevant check can be found here: https://github.com/Sandia-OpenSHMEM/SOS/blob/main/config/opal_check_pmi.m4#L284

Modifying this to check $pmix_ext_install_dir/lib64 resolves the configure failure. opal_check_pmi.m4 should be updated to check both lib and lib64 for libpmix.*.