Handle void results as unformatted prints

When printing void results without any format letter, they are output
as expected:
(gdb) p $abcd
$1 = void
(gdb) p (void)10
$2 = void

But if any format letter (besides s) is used, it always outputs zero:
(gdb) p/x $abcd
$3 = 0x0
(gdb) p/x (void)10
$4 = 0x0

So this adds void results to the types handled like unformatted prints.

gdb/ChangeLog:

2020-10-09  Hannes Domani  <ssbssa@yahoo.de>

	PR exp/26714
	* printcmd.c (print_formatted): Handle void results as
	unformatted prints.

gdb/testsuite/ChangeLog:

2020-10-09  Hannes Domani  <ssbssa@yahoo.de>

	PR exp/26714
	* gdb.base/printcmds.exp: Add tests for void results.