Compute a table of DWARF register sizes at compile

Compute a table of DWARF register sizes at compile

The sizes are compile-time constants.  Create a vector with them,
so that they can be inspected at compile time.

gcc/

	* dwarf2cfi.cc (init_return_column_size): Remove.
	(init_one_dwarf_reg_size): Adjust.
	(generate_dwarf_reg_sizes): New function.  Extracted
	from expand_builtin_init_dwarf_reg_sizes.
	(expand_builtin_init_dwarf_reg_sizes): Call
	generate_dwarf_reg_sizes.
	* target.def (init_dwarf_reg_sizes_extra): Adjust
	hook signature.
	* config/msp430/msp430.cc
	(msp430_init_dwarf_reg_sizes_extra): Adjust.
	* config/rs6000/rs6000.cc
	(rs6000_init_dwarf_reg_sizes_extra): Likewise.
	* doc/tm.texi: Update.