Make linking against libgcc configurable

Many (especially ARM) tool chains seem to come with broken or
otherwise unusable (for the purposes of builing U-Boot) run-time
support libraries `libgcc.a'. By using the "USE_PRIVATE_LIBGCC"
setting we allow to use alternative libraries instead.

"USE_PRIVATE_LIBGCC" can either be set as an environment variable in
the shell, or as a command line argument when running "make", i. e.
	$ make USE_PRIVATE_LIBGCC=yes
or
	$ USE_PRIVATE_LIBGCC=yes
	$ export USE_PRIVATE_LIBGCC
	$ make

The value of "USE_PRIVATE_LIBGCC" is the name of the directory which
contains the alternative run-time support library `libgcc.a'. The
special value "yes" selects the directory $(OBJTREE)/lib_$(ARCH) .

Note that not all architectures provide an alternative `libgcc.a' in
their lib_$(ARCH) directories - so far, only ARM does.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Stefan Roese <sr@denx.de>