Add test for EM _rck_. Fix traps in PowerPC ncg.

Add test for EM _rck_.  Fix traps in PowerPC ncg.

The new test rck_e.e segfaults on PowerPC unless I make some changes.
The inline code for _rck_ was wrong because it didn't allow the trap
handler to return.  _sig_ forgot to push the old trap handler.

Move plat/linuxppc/libsys/trap.s to mach/powerpc/libem/trp.s and
rewrite it with simplified/extended mnemonics.  Remove .trap alias for
.trp procedure.  Add a missing `mtspr lr, r0` so we can return from
the trap handler.  Call write() and _exit() so trp.s works with both
linuxppc and osxppc.  Before, Mac OS X was wrongly using the trap.s
for Linux.

In powerpc/libem, simplify .aar4; teach .csa and .csb to raise the
trap if the default target is zero.

C programs don't need these changes.  You may relink your C programs
with the changed .csa and .csb, but C code doesn't raise the trap.
Modula-2 code can raise traps, so you may want to relink your Modula-2
programs with the changed libem, but you might keep your old .o files
from Modula-2.  You may need to recompile your Pascal programs (delete
old .o files from Pascal) because the Pascal compiler might use _rck_.