Add target hook to override DWARF2 frame register size

Add target hook to override DWARF2 frame register size

gcc/

	* target.def (TARGET_DWARF_FRAME_REG_MODE): New target hook.
	* targhooks.c (default_dwarf_frame_reg_mode): New function.
	* targhooks.h (default_dwarf_frame_reg_mode): New prototype.
	* doc/tm.texi.in (TARGET_DWARF_FRAME_REG_MODE): Document.
	* doc/tm.texi: Regenerate.
	* dwarf2cfi.c (expand_builtin_init_dwarf_reg_sizes): Abstract mode
	selection logic to default_dwarf_frame_reg_mode.

From-SVN: r214898