Combine register lists of smp targets.

Combine register lists of smp targets.

This is helpful when you want to pretend to gdb that your heterogeneous
multicore system is homogeneous, because gdb cannot handle heterogeneous
systems. This won't always works, but works fine if e.g. one of the
cores has an FPU while the other does not. (Specifically, HiFive
Unleashed has 1 core with no FPU, plus 4 cores with an FPU.)

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: I05ff4c28646778fbc00327bc510be064bfe6c9f0
Reviewed-on: https://review.openocd.org/c/openocd/+/6362
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>