mips: rate limit the trap handler output; add pid/tid/program name.

I discovered that we're logging each trap, which gets pretty spendy;
and there wasn't any further information on the pid/tid/progname involved.

I originally noticed this because I don't attach anything to /dev/log and so
the log() output stays going to the kernel. That's an oops on my part, but
I'm glad I did it.

This commit adds the following:

• a rate limiter, which could do with some eyeballs/ideas on how to make it more predictable on SMP;
• log pid, tid, progname (comm) as part of the output.

I now get output like this:

Unaligned Load Word: pid=621 (pmcstat), tid=100060, pc=0xffffffff803ae898, badvaddr=0x40a10055
Unaligned Load Word: pid=621 (pmcstat), tid=100060, pc=0xffffffff803ae898, badvaddr=0x40a10051
Unaligned Load Word: pid=621 (pmcstat), tid=100060, pc=0xffffffff803ae898, badvaddr=0x40a1004d
Unaligned Load Word: pid=602 (login), tid=100042, pc=0xffffffff803ae898, badvaddr=0x401159
Unaligned Load Word: pid=602 (login), tid=100042, pc=0xffffffff803ae898, badvaddr=0x401155
Unaligned Load Word: pid=602 (login), tid=100042, pc=0xffffffff803ae898, badvaddr=0x401151

.. which makes it much easier to start figuring out what/where to fix.

The pc looks suss (it looks like it's in kernel space); I'll dig into that one next.

Tested:

• AR9331 SoC (Carambola2)