CFI-handling : Add a hook to allow target-specific Personality and LSDA indirections.

At present, the output of .cfi_personality and .cfi_lsda assumes
ELF semantics for indirections.  This isn't suitable for all targets
and is one blocker to moving Darwin to use .cfi_xxxx.

The patch adds a target hook that allows non-ELF targets to use
indirections appropriate to their needs.

gcc/ChangeLog:

	* config/darwin-protos.h (darwin_make_eh_symbol_indirect): New.
	* config/darwin.c (darwin_make_eh_symbol_indirect): New. Use
	Mach-O semantics for personality and ldsa indirections.
	* config/darwin.h (TARGET_ASM_MAKE_EH_SYMBOL_INDIRECT): New.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Add TARGET_ASM_MAKE_EH_SYMBOL_INDIRECT hook.
	* dwarf2out.c (dwarf2out_do_cfi_startproc): If the target defines
	a hook for indirecting personality and ldsa references, use that
	otherwise default to ELF semantics.
	* target.def (make_eh_symbol_indirect): New target hook.