selftests/nolibc: libc-test: avoid -Wstringop-overflow warnings

Newer versions of glibc annotate the poll() function with
__attribute__(access) which triggers a compiler warning inside the
testcase poll_fault.
Avoid this by using a plain NULL which is enough for the testcase.
To avoid potential future warnings also adapt the other EFAULT
testcases, except select_fault as NULL is a valid value for its
argument.

nolibc-test.c: In function ‘run_syscall’:
nolibc-test.c:338:62: warning: ‘poll’ writing 8 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
  338 |         do { if (!(cond)) result(llen, SKIPPED); else ret += expect_syserr2(expr, expret, experr1, experr2, llen); } while (0)
      |                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nolibc-test.c:341:9: note: in expansion of macro ‘EXPECT_SYSER2’
  341 |         EXPECT_SYSER2(cond, expr, expret, experr, 0)
      |         ^~~~~~~~~~~~~
nolibc-test.c:905:47: note: in expansion of macro ‘EXPECT_SYSER’
  905 |                 CASE_TEST(poll_fault);        EXPECT_SYSER(1, poll((void *)1, 1, 0), -1, EFAULT); break;
      |                                               ^~~~~~~~~~~~
cc1: note: destination object is likely at address zero
In file included from /usr/include/poll.h:1,
                 from nolibc-test.c:33:
/usr/include/sys/poll.h:54:12: note: in a call to function ‘poll’ declared with attribute ‘access (write_only, 1, 2)’
   54 | extern int poll (struct pollfd *__fds, nfds_t __nfds, int __timeout)
      |            ^~~~
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/testing/selftests/nolibc/nolibc-test.c

@@ -894,14 +894,14 @@ int run_syscall(int min, int max)
 		CASE_TEST(lseek_0);           EXPECT_SYSER(1, lseek(0, 0, SEEK_SET), -1, ESPIPE); break;
 		CASE_TEST(mkdir_root);        EXPECT_SYSER(1, mkdir("/", 0755), -1, EEXIST); break;
 		CASE_TEST(mmap_bad);          EXPECT_PTRER(1, mmap(NULL, 0, PROT_READ, MAP_PRIVATE, 0, 0), MAP_FAILED, EINVAL); break;
-		CASE_TEST(munmap_bad);        EXPECT_SYSER(1, munmap((void *)1, 0), -1, EINVAL); break;
+		CASE_TEST(munmap_bad);        EXPECT_SYSER(1, munmap(NULL, 0), -1, EINVAL); break;
 		CASE_TEST(mmap_munmap_good);  EXPECT_SYSZR(1, test_mmap_munmap()); break;
 		CASE_TEST(open_tty);          EXPECT_SYSNE(1, tmp = open("/dev/null", 0), -1); if (tmp != -1) close(tmp); break;
 		CASE_TEST(open_blah);         EXPECT_SYSER(1, tmp = open("/proc/self/blah", 0), -1, ENOENT); if (tmp != -1) close(tmp); break;
 		CASE_TEST(pipe);              EXPECT_SYSZR(1, test_pipe()); break;
 		CASE_TEST(poll_null);         EXPECT_SYSZR(1, poll(NULL, 0, 0)); break;
 		CASE_TEST(poll_stdout);       EXPECT_SYSNE(1, ({ struct pollfd fds = { 1, POLLOUT, 0}; poll(&fds, 1, 0); }), -1); break;
-		CASE_TEST(poll_fault);        EXPECT_SYSER(1, poll((void *)1, 1, 0), -1, EFAULT); break;
+		CASE_TEST(poll_fault);        EXPECT_SYSER(1, poll(NULL, 1, 0), -1, EFAULT); break;
 		CASE_TEST(prctl);             EXPECT_SYSER(1, prctl(PR_SET_NAME, (unsigned long)NULL, 0, 0, 0), -1, EFAULT); break;
 		CASE_TEST(read_badf);         EXPECT_SYSER(1, read(-1, &tmp, 1), -1, EBADF); break;
 		CASE_TEST(rmdir_blah);        EXPECT_SYSER(1, rmdir("/blah"), -1, ENOENT); break;
@@ -910,7 +910,7 @@ int run_syscall(int min, int max)
 		CASE_TEST(select_stdout);     EXPECT_SYSNE(1, ({ fd_set fds; FD_ZERO(&fds); FD_SET(1, &fds); select(2, NULL, &fds, NULL, NULL); }), -1); break;
 		CASE_TEST(select_fault);      EXPECT_SYSER(1, select(1, (void *)1, NULL, NULL, 0), -1, EFAULT); break;
 		CASE_TEST(stat_blah);         EXPECT_SYSER(1, stat("/proc/self/blah", &stat_buf), -1, ENOENT); break;
-		CASE_TEST(stat_fault);        EXPECT_SYSER(1, stat((void *)1, &stat_buf), -1, EFAULT); break;
+		CASE_TEST(stat_fault);        EXPECT_SYSER(1, stat(NULL, &stat_buf), -1, EFAULT); break;
 		CASE_TEST(stat_timestamps);   EXPECT_SYSZR(1, test_stat_timestamps()); break;
 		CASE_TEST(symlink_root);      EXPECT_SYSER(1, symlink("/", "/"), -1, EEXIST); break;
 		CASE_TEST(unlink_root);       EXPECT_SYSER(1, unlink("/"), -1, EISDIR); break;