tools/testing/selftests/sigaltstack/sas.c: improve output of sigaltstack testcase

tools/testing/selftests/sigaltstack/sas.c: improve output of sigaltstack testcase

Currently it uses %i for bitmasks, which makes it difficult to properly
decode the values.  Use %x instead.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Stas Sergeev <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>