Add option UNIFIED_OBJDIR, on by default, which moves the default build OBJDIR.

This changes the build OBJDIR from the older style of /usr/obj/<srcdir> for
native builds, and /usr/obj/<target>.<target_arch>/<srcdir> for cross builds to
a new simpler format of /usr/obj/<srcdir>/<target>.<target_arch>.  This
new format is used regardless of cross or native build.  It allows
easier management of multiple source tree object directories.

The UNIFIED_OBJDIR option will be removed and its feature made permanent
for the 12.0 release.

Relnotes:	yes (don't note UNIFIED_OBJDIR option since it will be removed)
Prior work:	D3711 D874
Reviewed by:	gjb, sjg
Discussed at:	https://lists.freebsd.org/pipermail/freebsd-arch/2016-May/017805.html
Discussed with:	emaste
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D12840

Makefile.libcompat

@@ -70,7 +70,7 @@ LIBSOFTWMAKEFLAGS=        -DCOMPAT_SOFTFP
 # Generic code for each type.
 # Set defaults based on type.
 libcompat=	${LIBCOMPAT:tl}
-_LIBCOMPAT_MAKEVARS=	_OBJTREE TMP CPUFLAGS CFLAGS CXXFLAGS WMAKEENV \
+_LIBCOMPAT_MAKEVARS=	_OBJTOP TMP CPUFLAGS CFLAGS CXXFLAGS WMAKEENV \
 			WMAKEFLAGS WMAKE
 .for _var in ${_LIBCOMPAT_MAKEVARS}
 .if !empty(LIB${LIBCOMPAT}${_var})
@@ -79,8 +79,8 @@ LIBCOMPAT${_var}?=	${LIB${LIBCOMPAT}${_var}}
 .endfor
 
 # Shared flags
-LIBCOMPAT_OBJTREE?=	${OBJTREE}${.CURDIR}/obj-lib${libcompat}
-LIBCOMPATTMP?=		${LIBCOMPAT_OBJTREE}/tmp
+LIBCOMPAT_OBJTOP?=	${OBJTOP}/obj-lib${libcompat}
+LIBCOMPATTMP?=		${LIBCOMPAT_OBJTOP}/tmp
 
 LIBCOMPATCFLAGS+=	${LIBCOMPATCPUFLAGS} \
 			-L${LIBCOMPATTMP}/usr/lib${libcompat} \
@@ -107,7 +107,8 @@ LIBCOMPATCXXFLAGS+=	-isystem ${LIBCOMPATTMP}/usr/include/c++/v1 -std=c++11 \
 .endif
 
 # Yes, the flags are redundant.
-LIBCOMPATWMAKEENV+= MAKEOBJDIRPREFIX=${LIBCOMPAT_OBJTREE} \
+LIBCOMPATWMAKEENV+= \
+		MAKEOBJDIRPREFIX=${LIBCOMPAT_OBJTOP} \
 		INSTALL="sh ${.CURDIR}/tools/install.sh" \
 		PATH=${TMPPATH} \
 		SYSROOT=${LIBCOMPATTMP} \
@@ -183,7 +184,8 @@ build${libcompat}: .PHONY
 	${_+_}cd ${.CURDIR}/${_dir}; \
 	    WORLDTMP=${WORLDTMP} \
 	    MAKEFLAGS="-m ${.CURDIR}/tools/build/mk ${.MAKEFLAGS}" \
-	    MAKEOBJDIRPREFIX=${LIBCOMPAT_OBJTREE} ${MAKE} SSP_CFLAGS= DESTDIR= \
+	    MAKEOBJDIRPREFIX=${LIBCOMPAT_OBJTOP} \
+	    ${MAKE} SSP_CFLAGS= DESTDIR= \
 	    DIRPRFX=${_dir}/ -DNO_LINT -DNO_CPU_CFLAGS MK_WARNS=no MK_CTF=no \
 	    build-tools
 .endfor