alloc.c: Import and export fewer symbols, and remove unused items.

alloc.c: Import and export fewer symbols, and remove unused items.

* lisp.h (suppress_checking, die): Declare only if ENABLE_CHECKING
is defined.
(suppress_checking): Add EXTERNALLY_VISIBLE attribute, so that
it's not optimized away by whole-program optimization.
(message_enable_multibyte, free_misc): Remove.
(catchlist, handlerlist, mark_backtrace):
Declare only if BYTE_MARK_STACK.
(mark_byte_stack): Likewise, fixing a ifdef-vs-if typo.
* alloc.c (pure): Export only if VIRT_ADDR_VARIES is defined.
(message_enable_multibyte): Remove decl.
(free_misc, interval_free_list, float_block, float_block_index):
(n_float_blocks, float_free_list, cons_block, cons_block_index):
(cons_free_list, last_marked_index):
Now static.
(suppress_checking, die): Define only if ENABLE_CHECKING is defined.
* eval.c (catchlist, handlerlist): Export only if BYTE_MARK_STACK.
(mark_backtrace): Define only if BYTE_MARK_STACK.
* xdisp.c (message_enable_multibyte): Now static.