Use casts to force an unsigned comparison in db_search_symbol().

On all of our platforms, db_expr_t is a signed integer while
db_addr_t is an unsigned integer value.  db_search_symbol used variables
of type db_expr_t to hold the current offset of the requested address from
the "best" symbol found so far.  This value was initialized to '~0'.
When a new symbol is found from a symbol table, the associated diff for the
new symbol is compared against the existing value as 'if (newdiff < diff)'
to determine if the new symbol had a smaller diff and was thus a closer
match.

On 64-bit MIPS, the '~0' was treated as a negative value (-1).  A lookup
that found a perfect match of an address against a symbol returned a diff
of 0.  However, in signed comparisons, 0 is not less than -1.  As a result,
DDB on 64-bit MIPS never resolved any addresses to symbols.  Workaround
this by using casts to force an unsigned comparison.

Probably the diff returned from db_search_symbol() and X_db_search_symbol()
should be changed to a db_addr_t instead of a db_expr_t as it is an
unsigned value (and is an offset of an address, so should fit in the same
size as an address).

Sponsored by:	DARPA / AFRL

sys/ddb/db_sym.c

@@ -373,10 +373,10 @@ db_search_symbol(db_addr_t val, db_strategy_t strategy, db_expr_t *offp)
 	register int	i;
 	c_db_sym_t	ret = C_DB_SYM_NULL, sym;
 
-	newdiff = diff = ~0;
+	newdiff = diff = val;
 	for (i = 0; i < db_nsymtab; i++) {
 	    sym = X_db_search_symbol(&db_symtabs[i], val, strategy, &newdiff);
-	    if (newdiff < diff) {
+	    if ((uintmax_t)newdiff < (uintmax_t)diff) {
 		db_last_symtab = &db_symtabs[i];
 		diff = newdiff;
 		ret = sym;