arc: Take into account the REGNUM in supply/collect gdb hooks
All the arc_linux_supply_*() target operations and the
arc_linux_collect_v2_regset() in arc-linux-tdep.c were
supplying/collecting all the registers in regcache as if the
REGNUM was set to -1.
The more efficient behavior is to examine the REGNUM and act
accordingly. That is what this patch does.