Linker script improvements

[kpgriesser]

The linker script, test/pim/rev-test-src/pim.lds, is initially provided to allow defining memory ranges that are decoded as MMIO by the memory controller. It needs to be enhanced for several reasons including:

1. Link to standard libraries. OK for now for basic demonstrations but we will need to build more sophisticated applications.

2. The REV loader is initializing the regions defined by MMIO space. Probably a simple directive will fix this. The initial writes to MMIO spaced are currently just being dropped with a warning message (verbose=3 for the PIM memory controller)

It would be nice if the MMIO symbols for dram, sram, and function space could be provided at the command line for g++ and avoid the LD step altogether.

There are probably more reasons to have a more sophisticated linker script.