HYPERV isn't available on all architectures, but just on by default

for i386/amd64. Rather, it only works on i386/amd64 and should only be
built there. Rather than change the default based on which
architecutre, do things more directly by only building it on
i386/amd64 and having it always on. This is how we handle other
options that are relevant only for a few architectures.

etc/devd/Makefile

@@ -8,6 +8,9 @@ FILES+=	apple.conf
 
 .if ${MACHINE} == "amd64" || ${MACHINE} == "i386"
 FILES+=	asus.conf
+.if ${MK_HYPERV} != "no"
+FILES+=	hyperv.conf
+.endif
 .endif
 
 .if ${MK_USB} != "no"
@@ -18,10 +21,6 @@ FILES+=	uath.conf ulpt.conf usb.conf
 FILES+=	zfs.conf
 .endif
 
-.if ${MK_HYPERV} != "no"
-FILES+=	hyperv.conf
-.endif
-
 NO_OBJ=
 FILESDIR=	/etc/devd
 FILESMODE=	644

libexec/Makefile

@@ -13,7 +13,6 @@ SUBDIR=	${_atf} \
 	fingerd \
 	ftpd \
 	getty \
-	${_hyperv} \
 	${_mail.local} \
 	${_mknetid} \
 	${_pppoed} \
@@ -55,10 +54,6 @@ _dma=		dma
 _dma-mbox-create=	dma-mbox-create
 .endif
 
-.if ${MK_HYPERV} != "no"
-_hyperv=	hyperv
-.endif
-
 .if ${MK_NIS} != "no"
 _mknetid=	mknetid
 _ypxfr=		ypxfr
@@ -95,4 +90,6 @@ _atf=		atf
 _tests=		tests
 .endif
 
+.include <bsd.arch.inc.mk>
+
 .include <bsd.subdir.mk>

libexec/Makefile.amd64

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

libexec/Makefile.i386

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

share/mk/src.opts.mk

@@ -90,6 +90,7 @@ __DEFAULT_YES_OPTIONS = \
     GPL_DTC \
     GROFF \
     HTML \
+    HYPERV \
     ICONV \
     INET \
     INET6 \
@@ -211,13 +212,6 @@ __DEFAULT_NO_OPTIONS+=CLANG_IS_CC CLANG CLANG_BOOTSTRAP
 __DEFAULT_YES_OPTIONS+=GCC GCC_BOOTSTRAP GNUCXX
 .endif
 
-# HyperV is only available for x86 and amd64.
-.if ${__T} == "amd64" || ${__T} == "i386"
-__DEFAULT_YES_OPTIONS+=HYPERV
-.else
-__DEFAULT_NO_OPTIONS+=HYPERV
-.endif
-
 .include <bsd.mkopt.mk>
 
 #