config.mk: use different host compiler for OS X 10.6

Compiling tools subdirectory on Mac OS X 10.6 (Snow Leopard) complains about
wrong syntax in system includes.

In file included from /usr/include/stdio.h:444,
                 from ../source/u-boot/include/compiler.h:26,
                 from ../source/u-boot/lib/crc32.c:15:
/usr/include/secure/_stdio.h:46: error: syntax error in macro parameter list

This can be fixed by reverting the workaround for prior OS X releases in
config.mk conditionally for OS X 10.6+.

Signed-off-by: Andreas Biemann <andreas.devel@googlemail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>