KVM: PPC: Book3S HV: Translate kvmhv_commence_exit to C

This replaces the assembler code for kvmhv_commence_exit() with C code
in book3s_hv_builtin.c.  It also moves the IPI sending code that was
in book3s_hv_rm_xics.c into a new kvmhv_rm_send_ipi() function so it
can be used by kvmhv_commence_exit() as well as icp_rm_set_vcpu_irq().

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>