Replace 'stack base' with 'stack bottom' in the documentation

Replace 'stack base' with 'stack bottom' in the documentation

* README.md (The C Interface to the Allocator): Outline that stack base
means its bottom; remove a note that client should set GC_stackbottom
on HP PA machines manually.
* doc/README.macros (USE_GET_STACKBASE_FOR_MAIN): Rename "stack base"
to "stack bottom".
* doc/porting.md (Adding Platforms to gcconfig.h): Likewise.
* doc/porting.md (Adding Platforms to gcconfig.h): Rename
"cool end of the stack" to "cold end of the stack"; refine
documentation about STACKBOTTOM.
* include/private/gcconfig.h (STACKBOTTOM): Rename "cool end" to
"cold end" in comment.
* include/gc.h (GC_stackbottom): Likewise.
* include/private/gcconfig.h (LINUX_STACKBOTTOM): Rename "stack base"
to "stack bottom" in comment.
* include/gc.h (GC_stack_base, GC_call_with_stack_base): Likewise.
* include/gc.h [GC_THREADS] (GC_register_my_thread): Likewise.
* misc.c [!THREADS] (GC_call_with_gc_active): Likewise.
* os_dep.c [LINUX_STACKBOTTOM] (GC_linux_main_stack_base): Likewise.
* os_dep.c (GC_get_stack_base): Likewise.
* pthread_support.c (GC_call_with_gc_active): Likewise.
* win32_threads.c (GC_call_with_gc_active): Likewise.