gdb/testsuite: Fix pretty-print.exp on big-endian platforms

gdb/testsuite: Fix pretty-print.exp on big-endian platforms

The pretty-print test case fails on s390/s390x because it relies on a
little-endian representation of bit fields.  Little-endian architectures
typically allocate bit fields from least to most significant bit, but
big-endian architectures typically use the reverse order, allocating the
most significant bit first.  Thus the two bit fields in each of the test
case's unions overlap either in their lower or in their higher bits,
depending on the target's endianness:

    union {
      int three : 3;
      int four : 4;
    };

Now, when initializing 'three' with 3, 'four' will become 3 on little
endian targets, but 6 on big-endian targets, making it FAIL there.

Fix this by initializing the longer bit field instead and using an
all-ones bit pattern.  In this way the result does not depend on
endianness.  Use 'unsigned' instead of int for one of the bit fields in
each of the unions, to increase the variety of resulting values.

gdb/testsuite/ChangeLog:

	* gdb.base/pretty-print.c (struct s1_t): Change fields 'three' and
	'six' to unsigned.
	(s1): Initialize fields 'four' and 'six' instead of 'three' and
	'five'.  Use an all-ones bit pattern for each.
	* gdb.base/pretty-print.exp: Adjust expected output of "print s1"
	to its changed values.