WIP get SPI echo test code working

Hi @dabelknap

I'm just collecting all the fixes I do to the SPI echo tests in this PR before merging, as there are some common issues you should be aware of.   Most of this is copying over bugs we killed on the CTP6 standalone code from the August integration test that I never ported to the other program.s

Infos practiques:

If suddenly the programs won't fit in the bram, you can set GCC to optimize for size instead of speed, see (da656b7).  

Note that when setting up the interrupts, you have to also enable the Xilinx exception table (0b4e89e).  I don't know why this is necessary, but it is.