Fix more typos in comments and documentation

Fix more typos in comments and documentation

* alloc.c: Fix typo ("overflow").
* configure.ac: Fix typo ("optimization").
* doc/README.amiga: Fix typos ("also", "specific", "unnecessary").
* doc/README.cords: Fix typos ("descendants", "highlighted").
* ChangeLog: Fix typos ("[un]collectible").
* README.md: Likewise.
* dbg_mlc.c: Likewise.
* doc/README.hp: Likewise.
* doc/README.linux: Likewise.
* doc/gcdescr.html: Likewise.
* doc/gcinterface.html: Likewise.
* include/gc_mark.h: Likewise.
* include/gc_pthread_redirects.h: Likewise.
* include/new_gc_alloc.h: Likewise.
* include/private/dbg_mlc.h: Likewise.
* include/private/gc_pmark.h: Likewise.
* include/private/gc_priv.h: Likewise.
* new_hblk.c: Likewise.
* ChangeLog: Fix typo ("multi-threaded").
* cord/cordxtra.c: Likewise.
* doc/debugging.html: Likewise.
* doc/gcinterface.html: Likewise.
* doc/leak.html: Likewise.
* doc/overview.html: Likewise.
* doc/simple_example.html: Likewise.
* include/gc_mark.h: Likewise.
* include/private/gc_priv.h: Likewise.
* os_dep.c: Likewise.
* ptr_chck.c: Likewise.
* README.md: Fix typo ("pointer-free").
* allchblk.c: Likewise.
* doc/debugging.html: Likewise.
* doc/gcdescr.html: Likewise.