gdb: change print format of flag enums with value 0

gdb: change print format of flag enums with value 0

If a flag enum has value 0 and the enumeration type does not have an
enumerator with value 0, we currently print:

  $1 = (unknown: 0x0)

I don't like the display of "unknown" here, since for flags, 0 is a
an expected value.  It just means that no flags are set.  This patch
makes it so that we print it as a simple 0 in this situation:

  $1 = 0

If there is an enumerator with value 0, it is still printed using that
enumerator, for example (from the test):

  $1 = FE_NONE

gdb/ChangeLog:

	* valprint.c (generic_val_print_enum_1): When printing a flag
	enum with value 0 and there is no enumerator with value 0, print
	just "0" instead of "(unknown: 0x0)".

gdb/testsuite/ChangeLog:

	* gdb.base/printcmds.exp (test_print_enums): Update expected
	output.