Give ddb more access to registers on macppc, powerpc64

Give ddb more access to registers on macppc, powerpc64

Edit db_regs[] in db_trace.c on both powerpc and powerpc64, so ddb can
access $r14, $r15, $r16, $dar, $dsisr.

Only for powerpc: change db_trap_glue to copy all registers to and
from ddb_regs (it was skipping some); change db_set_single_step and
db_clear_single_step to flip the correct bit of srr1; delete
FIXUP_PC_AFTER_BREAK, which was off by 1 instruction.

"ddb{1}> s" on my PowerMac7,3 (dual G5 at 2700 MHz) began to panic
like, "*cpu0: mutex 0xa7d0a0 not held in tc_update_timekeep".  Add an
arbitrary delay(100) after sending PPC_IPI_DDB; I want cpu0 to get the
ipi before it can see db_active == 1 and skip acquiring a mutex.

ok kettenis@