fix: Bash cross-compilation for i686-adros toolchain

fix: Bash cross-compilation for i686-adros toolchain

- build.sh: remove ulibc header install (conflicts with newlib POSIX headers)
- build.sh: add sys/ioctl.h, sys/termios.h, sys/dirent.h stubs to sysroot
- build.sh: add libgloss build step (crt0.o + libadros.a with posix_stubs)
- build.sh: add GCC specs file creation (--start-group/-lc/-ladros/--end-group)
- build.sh: comprehensive Bash cross-compile config.cache (types, headers, funcs)
- build.sh: use -D_POSIX_VERSION=200112L and --allow-multiple-definition
- syscalls.c: remove rename() (provided by newlib libc.a)
- posix_stubs.c: new file with POSIX function stubs for Bash linking
  (dup2, fcntl, pipe, select, pselect, tcgetattr, sigaction, sigprocmask,
   getcwd, waitpid, getpwnam, opendir, access, chdir, umask, sleep, etc.)

Bash 5.2.21 now cross-compiles as a static ELF 32-bit i386 binary.