bug #672: fixing pospopcount assembler

bug #672: fixing pospopcount assembler

there is a lot going on here, this is pushing the boundaries of
what ISAcaller can do (or hasnt been asked to do... until now)
* gbbd (gather bits and bytes double) had to be added
* sw=8,dw=64 had to be fixed (XLEN is actually 64 there
  but source elements have to be ZERO-EXTENDED...)
* a bug in sv.addi/sw=8 was found
  https://bugs.libre-soc.org/show_bug.cgi?id=1221
* some changes to setvl have to be made/written (!)
* sv.bc in CTR-reduction mode needs to potentially be fixed
  or at least properly examined