Make it possible to set CC and CXX (and optionally, AS and LD) in

make.conf(5), while allowing the build32 stage on 64-bit architectures
to still override them, so that stage can successfully build 32-bit
compatibility libraries.

Explanation:
1) The build32 stage sets environment variables CC, CXX, AS and LD for
   its sub-make, to add 32-bit specific flags (-m32 and such).
2) The sub-make reads sys.mk, encounters CC?= and CXX?= assignments, so
   does not alter them.
3) After some other stuff, sys.mk reads /etc/make.conf.  When you have
   "CC=xxx" and "CXX=yyy" statements in there, they will *override* the
   build32-supplied CC/CXX values, nullifying the 32-bit specific flags.
4) Thus all objects get built as 64-bit anyway, and since LD is usually
   not set in make.conf, it still has the 32-bit flags!
5) Now, whenever something is linked, you will get a "ld: Relocatable
   linking with relocations from format elf64-x86-64-freebsd (foo.o) to
   format elf32-i386-freebsd (bar.o) is not supported" error.

Fix this by adding "-ECC -ECXX -EAS -ELD" to the build32 sub-make
invocation, which forces those environment variables to always override
any assignment in makefiles.  Thus making it possible to simply set:

CC=my-cc
CXX=my-c++

in your make.conf, or specify a path, even:

CC=/usr/local/bin/other-cc
CXX=/usr/local/bin/other-c++

Note this was never a problem on i386, since it has no build32 stage.

Silence from:	current@
MFC after:	1 week

Makefile.inc1

@@ -313,7 +313,8 @@ LIB32WMAKEENV+=	MAKEOBJDIRPREFIX=${OBJTREE}/lib32 \
 
 LIB32WMAKE=	${LIB32WMAKEENV} ${MAKE} -DNO_CPU_CFLAGS -DCOMPAT_32BIT \
 		-DWITHOUT_BIND -DWITHOUT_MAN -DWITHOUT_INFO \
-		-DWITHOUT_HTML -DNO_CTF -DNO_LINT DESTDIR=${LIB32TMP}
+		-DWITHOUT_HTML -DNO_CTF -DNO_LINT -ECC -ECXX -EAS -ELD \
+		DESTDIR=${LIB32TMP}
 LIB32IMAKE=	${LIB32WMAKE:NINSTALL=*:NDESTDIR=*} -DNO_INCS
 .endif