tools: fix testing/selftests/sigaltstack for s390x

On s390x the compilation of the file sas.c in directory
tools/testing/selftests/sigaltstack fails with this error message:

   root@s35lp76 testing]# make selftests/sigaltstack/sas
   cc     selftests/sigaltstack/sas.c   -o selftests/sigaltstack/sas
   selftests/sigaltstack/sas.c: In function ‘my_usr1’:
   selftests/sigaltstack/sas.c:42:25: error: invalid register name for ‘sp’
     register unsigned long sp asm("sp");
                         ^~
   <builtin>: recipe for target 'selftests/sigaltstack/sas' failed
   make: *** [selftests/sigaltstack/sas] Error 1
   [root@s35lp76 testing]#

On s390x the stack pointer is register r15, the register name "sp"
is unknown.
Make this line platform dependend and use register r15.

With this patch the compilation and test succeeds:

   [root@s35lp76 testing]# ./selftests/sigaltstack/sas
   TAP version 13
   ok 1 Initial sigaltstack state was SS_DISABLE
   # [RUN]	signal USR1
   ok 2 sigaltstack is disabled in sighandler
   # [RUN]	switched to user ctx
   # [RUN]	signal USR2
   # [OK]	Stack preserved
   ok 3 sigaltstack is still SS_AUTODISARM after signal
   Pass 3 Fail 0 Xfail 0 Xpass 0 Skip 0 Error 0
   1..3
   [root@s35lp76 testing]#
Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/sigaltstack/sas.c

@@ -39,7 +39,11 @@ void my_usr1(int sig, siginfo_t *si, void *u)
 	stack_t stk;
 	struct stk_data *p;
 
+#if __s390x__
+	register unsigned long sp asm("%15");
+#else
 	register unsigned long sp asm("sp");
+#endif
 
 	if (sp < (unsigned long)sstack ||
 			sp >= (unsigned long)sstack + SIGSTKSZ) {