i#1973 musl: Don't pass SS_ONSTACK to sigaltstack in tests (#7198)

POSIX.1 requires SS_ONSTACK to appear only in a stack_t retrieved from
sigaltstack(), not one passed to it. musl checks the invalid flag and
will
bail out with EINVAL before issuing a syscall in this case, failing the
test.

This commit sets ss_flags to zero instead, on both glibc and musl this
should issue a syscall, which fixes assertion failures encountered in
linux.signal*, api.static_signal and drcachesim.burst_threads on musl.

Issue: #1973

clients/drcachesim/tests/burst_threads.cpp

@@ -107,7 +107,7 @@ void *
 #    define ALT_STACK_SIZE (SIGSTKSZ * 2)
     sigstack.ss_sp = (char *)malloc(ALT_STACK_SIZE);
     sigstack.ss_size = ALT_STACK_SIZE;
-    sigstack.ss_flags = SS_ONSTACK;
+    sigstack.ss_flags = 0;
     int res = sigaltstack(&sigstack, NULL);
     assert(res == 0);
 #endif

suite/tests/api/static_signal.c

@@ -85,7 +85,7 @@ thread_func(void *arg)
     int rc;
     sigstack.ss_sp = (char *)malloc(ALT_STACK_SIZE);
     sigstack.ss_size = ALT_STACK_SIZE;
-    sigstack.ss_flags = SS_ONSTACK;
+    sigstack.ss_flags = 0;
     rc = sigaltstack(&sigstack, NULL);
     ASSERT_NOERR(rc);
     signal_cond_var(thread_ready);

suite/tests/linux/bad-signal-stack.c

@@ -89,7 +89,7 @@ main(int argc, char *argv[])
     /* Make an alternate stack that's not writable. */
     sigstack.ss_sp = alloc;
     sigstack.ss_size = ALT_STACK_SIZE;
-    sigstack.ss_flags = SS_ONSTACK;
+    sigstack.ss_flags = 0;
     rc = sigaltstack(&sigstack, NULL);
     ASSERT_NOERR(rc);
     protect_mem((void *)sigstack.ss_sp, ALT_STACK_SIZE, ALLOW_READ);

suite/tests/linux/signal-base.h

@@ -124,7 +124,7 @@ static void
     stack_t sigstack;
     sigstack.ss_sp = siginfo; /* will fail: just need sthg */
     sigstack.ss_size = ALT_STACK_SIZE;
-    sigstack.ss_flags = SS_ONSTACK;
+    sigstack.ss_flags = 0;
     int rc = sigaltstack(&sigstack, NULL);
     assert(rc == -1 && errno == EPERM);
 #endif