refactor: move sysroot compat headers into project + update build.sh

refactor: move sysroot compat headers into project + update build.sh

All 71 Linux/POSIX compatibility headers that were previously created
directly in /opt/adros-toolchain/i686-adros/include/ are now stored
in newlib/sysroot_headers/ and installed by build.sh via cp -r.

Headers include: asm/*, linux/*, net/*, netinet/*, netpacket/*,
sys/socket.h, sys/mman.h, sys/mount.h, sys/epoll.h, poll.h, netdb.h,
mntent.h, paths.h, shadow.h, syslog.h, utmp.h, endian.h, byteswap.h,
features.h, dlfcn.h, ifaddrs.h, arpa/telnet.h, and more.

Also moved existing inline heredoc headers (sys/ioctl.h, sys/termios.h,
sys/dirent.h) from build.sh into sysroot_headers/ for consistency.

Newlib header patches (sys/stat.h, sys/signal.h, sys/wait.h, glob.h)
are now applied via idempotent sed commands in build.sh:
- sys/stat.h: expose lstat()/mknod() for __adros__
- sys/signal.h: add SA_RESTART/SA_NODEFER/SA_RESETHAND/SA_NOCLDWAIT
- sys/wait.h: add WCOREDUMP macro
- glob.h: add GLOB_NOMATCH

build.sh also updated to compile posix_compat.c into libadros.a.

Tests: 101/101 smoke, cppcheck clean