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

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