sim/riscv: fix JALR instruction simulation

Fix 32bit 'jalr rd,ra,imm' integer instruction, where RD was written
before using it to calculate destination address.

This commit also improves testutils.inc for riscv; make use of
pushsection and popsection when adding things to .data, and setup the
%gp global pointer register within the 'start' macro.

Approved-By: Andrew Burgess <aburgess@redhat.com>