gdb: move store/extract integer functions to extract-store-integer.{c,h}

Move the declarations out of defs.h, and the implementations out of
findvar.c.

I opted for a new file, because this functionality of converting
integers to bytes and vice-versa seems a bit to generic to live in
findvar.c.

Change-Id: I524858fca33901ee2150c582bac16042148d2251
Approved-By: John Baldwin <jhb@FreeBSD.org>

gdb/fbsd-tdep.c

@@ -18,6 +18,7 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "auxv.h"
+#include "extract-store-integer.h"
 #include "gdbcore.h"
 #include "inferior.h"
 #include "objfiles.h"