Previously, cc/gcc.sh assumed that liblto_plugin would always be
installed with the ".so" file extension. However, this assumption is
incorrect when the host machine is Windows (".dll") or MacOS (".dylib").
This patch corrects this issue by probing the file extension in similar
fashion to the way adjacent code determines the file extension of
executable binaries.
Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>