Lots of RISC-V improvements.

Lots of RISC-V improvements.

This represents months of continuing RISC-V work, with too many changes
to list individually. Some improvements:
* Fixed memory leaks.
* Better handling of dbus timeouts.
* Add `riscv expose_custom` command.
* Somewhat deal with cache coherency.
* Deal with more timeouts during block memory accesses.
* Basic debug compliance test.
* Tell gdb which watchpoint hit.
* SMP support for use with -rtos hwthread
* Add `riscv set_ir`

Change-Id: Ica507ee2a57eaf51b578ab1d9b7de71512fdf47f
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4922
Tested-by: jenkins
Reviewed-by: Philipp Guehring <pg@futureware.at>
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>