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/ada-lang.c

@@ -19,6 +19,7 @@
 
 
 #include <ctype.h>
+#include "extract-store-integer.h"
 #include "gdbsupport/gdb_regex.h"
 #include "frame.h"
 #include "symtab.h"