[PATCH] Darwin dead code strip support

[Devang Patel <dpatel at apple dot com>]

Now, Darwin linker supports dead code strip. Which means, at link time, 
linker can eliminate unused symbols. But sometimes user may want to 
keep some of them (for example debug_tree() in gcc). This patch allows 
user to annotate such symbols using existing attribute used. Target 
hook is used to allow various targets to take appropriate actions when 
attribute used is used. Darwin target hook emits .no_dead_strip 
assembler directive.

Bootstrapped and tested on powerpc-darwin.

OK?
Thanks,
-
Devang

2004-08-11  Devang Patel  <dpatel@apple.com>

        * c-common.c (handle_used_attribute): Let target mark decl used.
        * c-decl.c (start_function): decls decorated with attribute used
        may already have their assembler name set.
        * target-def.h (TARGET_ASM_MARK_DECL_USED): New #define.
        (TARGET_ASM_OUT): New member, TARGET_ASM_MARK_DECL_USED.
        * target.h (struct gcc_target): New member, mark_decl_used.
        * targhooks.c (default_mark_decl_used): New function.
        * targhooks.h (default_mark_decl_used): New decl.
        * darwin.c (darwin_mark_decl_used): New function.
        * darwin.h (TARGET_ASM_MARK_DECL_USED): New #define.

        testsuite:
        gcc.dg/darwin-20040809-1.c: New test

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