Allow ignoring the native cache within a pkgimage.

Currently --pkgimage=no triggers the recreation of a new cache-file. So does a couple of other Julia options. While for certain options this bifurcation of cache files might make sense, for others it's just duplicated work.

We could move the .data section back out of the .so and back into the .ji file, so allowing us to just load the .ji file and disregard the .so, or simply ignore the .text section and not wire up the native cache.

@gbaraldi had I think a couple of aborted attempts, but I don't think either of us currently have the time.