feat: Phase 5 — posix_compat.c + Busybox 1.36.1 cross-compilation

feat: Phase 5 — posix_compat.c + Busybox 1.36.1 cross-compilation

Add posix_compat.c to libgloss/adros with comprehensive POSIX stubs
needed for cross-compiling Busybox and other ported software:
- poll() via select() emulation
- popen/pclose stubs
- getline/getdelim
- dirname/basename (libgen)
- symlink/lchown/utimes/ftruncate stubs
- getgroups/setpriority/getpriority
- getrlimit/setrlimit (RLIM_INFINITY defaults)
- mount table: setmntent/getmntent/endmntent/getmntent_r
- mount/umount2 stubs
- DNS stubs: getaddrinfo/freeaddrinfo/gai_strerror/getnameinfo/
  gethostbyname/hstrerror/h_errno
- mknod/execvp/sysinfo/getrusage
- signal() via sigaction wrapper
- uname() syscall (INT 0x80, nr=136)
- sigsuspend/vfork (fork emulation)
- mmap (MAP_ANONYMOUS via malloc)/munmap
- rmdir (INT 0x80, nr=40)
- chroot/readlink/realpath/sysconf/fchown stubs
- clock_settime/utimensat stubs
- clearenv/getgrnam/getgrgid stubs
- sethostname/inet_ntoa/htonl/htons/ntohl/ntohs

Busybox 1.36.1 cross-compiled successfully:
- 262KB static ELF32 i686-adros, 70 applets
- Includes: ash shell, cat, ls, cp, mv, rm, mkdir, chmod, chown,
  grep, sed, diff, find, xargs, sort, head, tail, wc, cut, tr,
  printf, echo, env, sleep, date, dd, stat, readlink, hexdump,
  md5sum, sha256sum, hostname, kill, clear, more, and more

Tests: 101/101 smoke, 64/64 host utils, cppcheck clean