bhyve: Move AP startup code to amd64/

This code is only invoked via MD vmexit handlers.  No functional change
intended.

Reviewed by:	corvink, jhb
MFC after:	1 week
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D40737

usr.sbin/bhyve/Makefile

@@ -62,7 +62,6 @@ SRCS=	\
 	rfb.c			\
 	smbiostbl.c		\
 	sockstream.c		\
-	spinup_ap.c		\
 	tpm_device.c		\
 	tpm_emul_passthru.c	\
 	tpm_intf_crb.c		\

usr.sbin/bhyve/amd64/Makefile.inc

@@ -10,6 +10,7 @@ SRCS+=	\
 	ps2kbd.c	\
 	ps2mouse.c	\
 	rtc.c		\
+	spinup_ap.c	\
 	task_switch.c	\
 	xmsr.c