Fix dtc bugs for 64-bit compile

I've just tested building dtc as an x86_64 binary on a 32-bit i386
host by using:
	make CC="gcc -m64"
This patch fixes a handful of minor bugs thus discovered:

 * There is a printf() type mismatch on 64-bit in value-labels.c

 * For the tests which use libdl, we were using the GNU make feature
where it will find libdl.so given a dependency in the form '-ldl'.
But this built-in make logic doesn't know we're compiling 64-bit so
finds the 32-bit version of the library.  We avoid using this and
instead explicitly pass -ldl to CC, which being the 64-bit version
does know where to look.

 * To process dtc's asm output into .so files, run_tests.sh was
directly invoking the (default instance of) the assembler and linker.
Instead invoke these via the CC driver, and allow that to be overriden
from the make environment.

 * The x86_64 assembler doesn't 0 fill with the .balign directive
(presumably it is NOP filling).  That doesn't produce strictly
incorrect trees, but it is confusing and confounds are testcases which
do byte-by-byte comparison of the trees produced by asm output with
direct dtb output (which does 0 pad where necessary, of course).  This
patch uses the optional second argument to .balign to force gas to
zero-fill instead.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/run_tests.sh

@@ -2,6 +2,10 @@
 
 . ./tests.sh
 
+if [ -z "$CC" ]; then
+    CC=gcc
+fi
+
 export QUIET_TEST=1
 
 export VALGRIND=
@@ -72,8 +76,7 @@ run_dtc_test () {
 }
 
 asm_to_so () {
-    as -o $1.test.o data.S $1.test.s && \
-	ld -shared -o $1.test.so $1.test.o
+    $CC -shared -o $1.test.so data.S $1.test.s
 }
 
 asm_to_so_test () {