gc, pal: Avoid linking libnuma.so on non-NUMA systems (#1468)

gc, pal: Avoid linking libnuma.so on non-NUMA systems (#1468)

* gc, pal: Avoid linking libnuma.so on non-NUMA systems

In an effort to reduce the system calls performed during the
initialization, this seemed like another low-hanging fruit: only link
in libnuma.so if the system has more than one NUMA node.

* gc, pal: Remove unneeded dlsym() call

The result of dlsym() is never attributed to any variable, and this
function is never called, so it's safe to remove these dlsym() calls.