DynaLoader: Pure-perl mod2fname shouldn't use %Config at runtime

This is somewhat obscure.  Previously, mod2fname would've tried
accessing $Config{dlext} each time it was called.  The reason this
can be problematic is that mod2fname can be called during global destruction,
after %Config is freed but before its magic is.  This was causing
Devel::GlobalDestruction's tests to fail on builds with -Dd_libname_unique.

This commit grabs the data we need when DynaLoader.pm is being built in
DynaLoader_pm.PL, so it no longer need to depend on %Config.