amd64: Only build aout.ko when COMPAT_FREEBSD32 is enabled

a.out.ko is useless without COMPAT_FREEBSD32, and since vdso support was
added, it requires a header built during depend phase, from the COMPAT
option.

Reported by:	glebius
Reviewed by:	emaste, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D33311

sys/conf/kern.post.mk

@@ -50,6 +50,10 @@ MKMODULESENV+=	SAN_CFLAGS="${SAN_CFLAGS}"
 MKMODULESENV+=	GCOV_CFLAGS="${GCOV_CFLAGS}"
 .endif
 
+.if !empty(COMPAT_FREEBSD32_ENABLED)
+MKMODULESENV+=	COMPAT_FREEBSD32_ENABLED="yes"
+.endif
+
 # Allow overriding the kernel debug directory, so kernel and user debug may be
 # installed in different directories. Setting it to "" restores the historical
 # behavior of installing debug files in the kernel directory.

sys/conf/kern.pre.mk

@@ -93,6 +93,8 @@ CFLAGS+=	-fno-common
 # XXX LOCORE means "don't declare C stuff" not "for locore.s".
 ASM_CFLAGS= -x assembler-with-cpp -DLOCORE ${CFLAGS} ${ASM_CFLAGS.${.IMPSRC:T}}
 
+COMPAT_FREEBSD32_ENABLED!= grep COMPAT_FREEBSD32 opt_global.h || true ; echo
+
 KASAN_ENABLED!=	grep KASAN opt_global.h || true ; echo
 .if !empty(KASAN_ENABLED)
 SAN_CFLAGS+=	-DSAN_NEEDS_INTERCEPTORS -DSAN_INTERCEPTOR_PREFIX=kasan \

sys/modules/Makefile

@@ -623,7 +623,9 @@ _neta=		neta
 
 .if ${MACHINE_CPUARCH} == "i386" || ${MACHINE_CPUARCH} == "amd64"
 _agp=		agp
+.if ${MACHINE_CPUARCH} == "i386" || !empty(COMPAT_FREEBSD32_ENABLED)
 _aout=		aout
+.endif
 _bios=		bios
 .if ${MK_SOURCELESS_UCODE} != "no"
 _bxe=		bxe