Remove references to missing linux_threads.c from documentation

Remove references to missing linux_threads.c from documentation

* README.md (copyright): Change linux_threads.c to pthread_stop_world.c
and pthread_support.c files.
* doc/README.DGUX386: Remove reference to linux-threads.c file.
* doc/README.environment (GC_RETRY_SIGNALS, GC_NO_RETRY_SIGNALS):
Likewise.
* doc/gcdescr.html: Likewise.
* doc/README.DGUX386: Update information how to pass DEBUG_THREADS to
make.
* doc/gcdescr.html: Update threads support information for Irix and
Solaris.
* include/private/gc_locks.h: Remove reference to linux-threads.c in
a comment.
* pthread_support.c: Likewise.