sysdeps/managarm: SignalGuard assertion failure while running configure on Managarm

[Dennisbonke]

While running configure for GNU Sed, on the dup2 check, the following assert is tripped.

posix: Ignoring unexpected value 16 of global signal flag
In function ~SignalGuard, file ../../../src/mlibc/sysdeps/managarm/generic/entry.cpp:74
__ensure(result == 1) failed

The check that configure is executing is pasted below

int
main (void)
{
int result = 0;
           int bad_fd = INT_MAX;
           struct rlimit rlim;
           if (getrlimit (RLIMIT_NOFILE, &rlim) == 0
               && 0 <= rlim.rlim_cur && rlim.rlim_cur <= INT_MAX
               && rlim.rlim_cur != RLIM_INFINITY
               && rlim.rlim_cur != RLIM_SAVED_MAX
               && rlim.rlim_cur != RLIM_SAVED_CUR)
             bad_fd = rlim.rlim_cur;
           #ifdef FD_CLOEXEC
             if (fcntl (1, F_SETFD, FD_CLOEXEC) == -1)
               result |= 1;
           #endif
           if (dup2 (1, 1) != 1)
             result |= 2;
           #ifdef FD_CLOEXEC
             if (fcntl (1, F_GETFD) != FD_CLOEXEC)
               result |= 4;
           #endif
           close (0);
           if (dup2 (0, 0) != -1)
             result |= 8;
           /* Many gnulib modules require POSIX conformance of EBADF.  */
           if (dup2 (2, bad_fd) == -1 && errno != EBADF)
             result |= 16;
           /* Flush out some cygwin core dumps.  */
           if (dup2 (2, -1) != -1 || errno != EBADF)
             result |= 32;
           dup2 (2, 255);
           dup2 (2, 256);
           /* On OS/2 kLIBC, dup2() does not work on a directory fd.  */
           {
             int fd = open (".", O_RDONLY);
             if (fd == -1)
               result |= 64;
             else if (dup2 (fd, fd + 1) == -1)
               result |= 128;
             close (fd);
           }
           return result;
  ;
  return 0;
}