cmake/isal-l: explicitly configure libdir to avoid wrong libisal.a

cmake/isal-l: explicitly configure libdir to avoid wrong libisal.a
location.

This is to fix the following libec_isa.si build error :

FAILED: lib/libec_isa.so
: && /usr/bin/g++-14 -fPIC -O3 -DNDEBUG
-Wl,--dependency-file=src/erasure-code/isa/CMakeFiles/ec_isa.dir/link.d
-shared -Wl,-soname,libec_isa.so -o lib/libec_isa.so
src/erasure-code/CMakeFiles/erasure_code_objs.dir/ErasureCode.cc.o
src/erasure-code/isa/CMakeFiles/ec_isa.dir/ErasureCodeIsa.cc.o
src/erasure-code/isa/CMakeFiles/ec_isa.dir/ErasureCodeIsaTableCache.cc.o
src/erasure-code/isa/CMakeFiles/ec_isa.dir/ErasureCodePluginIsa.cc.o
src/isa-l/install/lib/libisal.a  -ldl  /usr/lib64/librt.a  -lresolv
-Wl,--as-needed -latomic && :
/usr/lib64/gcc/x86_64-suse-linux/14/../../../../x86_64-suse-linux/bin/ld:
cannot find src/isa-l/install/lib/libisal.a: No such file or directory
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

It looks like [under some circumstances?] the building procedure puts
resulting .a under src/isa-l/install/lib64 path which causes lookup
error .so linkage.

This patch enforces lib64 usage.

Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>