hyperv: Build and install where supported

Remove the hard-coded dependency on HYPERV being only x86. Instead, 100%
rely on MK_HYPERV. It's always right (since it's marked BROKEN (so set
to "no") on architectures we don't support).

Sponsored by:		Netflix
Reviewed by:		bz
Differential Revision:	https://reviews.freebsd.org/D38306

lib/libc/x86/sys/Makefile.inc

@@ -10,6 +10,7 @@ SRCS+= \
 MAN+=	\
 	pkru.3
 
+# Note: vdso support for hyperv only on amd64
 .if ${MACHINE_CPUARCH} == "amd64" && ${MK_HYPERV} != "no"
 CFLAGS+=	-DWANT_HYPERV
 .endif

libexec/Makefile

@@ -10,6 +10,7 @@ SUBDIR=	${_atf} \
 	${_dma} \
 	flua \
 	getty \
+	${_hyperv} \
 	${_mail.local} \
 	${_makewhatis.local} \
 	${_mknetid} \
@@ -64,6 +65,10 @@ _comsat=	comsat
 _dma=		dma
 .endif
 
+.if ${MK_HYPERV} != "no"
+_hyperv+=	hyperv
+.endif
+
 .if ${MK_NIS} != "no"
 _mknetid=	mknetid
 _ypxfr=		ypxfr

libexec/Makefile.amd64

@@ -1,5 +0,0 @@
-# $FreeBSD$
-
-.if ${MK_HYPERV} != "no"
-SUBDIR+=	hyperv
-.endif

libexec/Makefile.i386

@@ -1,5 +0,0 @@
-# $FreeBSD$
-
-.if ${MK_HYPERV} != "no"
-SUBDIR+=	hyperv
-.endif

usr.sbin/Makefile

@@ -140,6 +140,7 @@ SUBDIR.${MK_FLOPPY}+=	fdwrite
 SUBDIR.${MK_FREEBSD_UPDATE}+=	freebsd-update
 SUBDIR.${MK_GSSAPI}+=	gssd
 SUBDIR.${MK_GPIO}+=	gpioctl
+SUBDIR.${MK_HYPERV}+=	hyperv
 SUBDIR.${MK_INET6}+=	ip6addrctl
 SUBDIR.${MK_INET6}+=	mld6query
 SUBDIR.${MK_INET6}+=	ndp

usr.sbin/Makefile.amd64

@@ -23,9 +23,6 @@ SUBDIR+=	boot0cfg
 SUBDIR+=	btxld
 .endif
 SUBDIR+=	cpucontrol
-.if ${MK_HYPERV} != "no"
-SUBDIR+=	hyperv
-.endif
 SUBDIR+=	lptcontrol
 SUBDIR+=	mptable
 SUBDIR+=	spkrtest

usr.sbin/Makefile.i386

@@ -12,9 +12,6 @@ SUBDIR+=	boot0cfg
 SUBDIR+=	btxld
 .endif
 SUBDIR+=	cpucontrol
-.if ${MK_HYPERV} != "no"
-SUBDIR+=	hyperv
-.endif
 SUBDIR+=	lptcontrol
 SUBDIR+=	mptable
 SUBDIR+=	pnpinfo