this is intermittently working, inexplicably. reason unknown: it could be down to how gnu-as disassembles and identifies mtocrf/mtcrf, it could be timing-related on decoding of the appropriate bit (bit 11 aka bit 20).
https://git.libre-soc.org/?p=soc.git;a=blob;f=src/soc/decoder/isa/caller.py;h=8b40a1c821538819eb0a232c8d580f90ad6879a5;hb=HEAD#l456