gdb: print unknown part of flag enum in hex

When we print the "unknown" part of a flag enum, it is printed in
decimal.  I think it would be more useful if it was printed in hex, as
it helps to determine which bits are set more than a decimal value.

gdb/ChangeLog:

	* valprint.c (generic_val_print_enum_1): Print unknown part of
	flag enum in hex.

gdb/testsuite/ChangeLog:

	* gdb.base/printcmds.exp (test_print_enums): Expect hex values
	for "unknown".