For macppc, add missing CFI for cr2, cr3, cr4

For macppc, add missing CFI for cr2, cr3, cr4

clang -S was missing a line like ".cfi_offset cr2, -16" in functions
that spill cr2 (or cr3, cr4) to the stack.  This was breaking a few
C++ exceptions.  This fix adds the missing CFI.

This will fix the C++ exception that was crashing egdb from
ports/devel/gdb when you did "q to quit" in the pager.

ok kettenis@ tobhe@