Clear BTYPE bits when setting up a signal handler and when handling a

Clear BTYPE bits when setting up a signal handler and when handling a
PT_CONTINUE ptrace(2) request.  Otherwise we would trap if userland was
interrupted at a point where it is doing an indirect branch that has set
the bits but before it has executed the BTI instruction at the branch
target.

The PT_SETREGS request may need similar treatment, at least when the
PC is changed.  But Linux doesn't do this and debuggers might want full
control over the BTYPE bits.  So leave this alone for now.

ok guenther@