Changes to make MIPS x 29K compile.

Changes to make MIPS x 29K compile.

* am29k-tdep.c:  Lint from DECstation compiler.
* mem-break.c:  Restore test of BREAKPOINT size that Fred removed.
As the man page says, "Sometimes you just can't get lint to shut
up".  That doesn't mean you should blow away the code it won't
shut up about.
* mips-xdep.c:  Revise stubbing-out of code until Rich cleans
it up over the next few weeks.  Make mips x 29k build.
* tm-29k.h (CALL_DUMMY):  Make it work on cross-endian hosts.
(FIX_CALL_DUMMY):  Comment in the patching of the breakpoint,
but leave it as a comment because the breakpoint instruction
is not easily accessible at this moment (it's static, and if
we define one here, it goes into every file compiled).