DTrace/MIPS port

Hello,

Last few weeks I've been working on DTrace port for MIPS architecture.
I believe that project reached the stage when it's ready for public
review/testing before going into the tree.

Patch and some information could be found here:
http://people.freebsd.org/~gonzo/mips/dtrace/

I'd appreciate review/testing from interested parties and if there are
no major roadblocks the plan is to commit this patch sometime next
week.

DTrace/MIPS passes substantial part of DTrace suite on my
Octeon-based board.

==== TEST RESULTS ====

      mode: /usr/sbin/dtrace
    passed: 853
    failed: 74
     total: 927

There are some caveats/limitations though:

- fbt, pid, lockstat, profile providers are not implemented

- MIPS passes function arguments in registers and unless they're
     saved on stack the value of some might be unavailable in
     backtrace. So values of argN variables might be bogus sometimes.

- dtrace uses kldstat(2) to get path to kernel binary and for
     "embedded" systems (e.g. without loader(8)) it's just "kernel"
     So kernel binary should be in current directory so dtrace could
     get CTF data from it. We need either command-line switch or env
     variable to let dtrace know where to look for binary. I haven't
     yet decided which way to go.

- Not really dtrace issue, but somewhat related. FreeBSD/MIPS default
     kernel stacks size seems to be insufficient to load kernel
     modules with dependency chain longer then three modules
     (dtrace_test -> dtrace_all -> dtrace -> cyclic -> opensolaris)
     Sometimes I get kernel stack exhaustion as a combination of
     FS-related calls that goes down to NFS functions + WITNESS code.
     No proper solution for it yet. Workaround - load module one by
     one.

- Tested only on mips64be platform. mips32be, mips32le, mips64le
     were not tested.

Patches:

dtrace-all.diff - is a cumulative patch that contains diff between
HEAD branch and project branch in p4. In order to make code review
easier I split it into several sub-patches based on functionality area.

dtrace-ctf.diff
     Current version of ctfmerge assumes that target byte order is the
     same as host one. This patch checks byte order of ELF files being
     used to decide whether byte order in CTF structures' fields
     should be reversed.

dtrace-toolchain.diff
     - Disable SGI compatibility for generated DWARF data.
           It confuses ctfconvert.
     - Set as(1) default ABI and target size the same as target platform

dtrace-sys.diff
     - Kernel part of DTrace code
     - More intelligent kernel stack overflow handler

dtrace-userland.diff
     - Userland part of DTrace code
     - Build DTrace tols as a part of toolchain build if
         we're cross-compiling
     - Various libraries' plugs for MIPS