Define __LIBGCC_DWARF_REG_SIZES_CONSTANT__ if DWARF register size is constant

Define __LIBGCC_DWARF_REG_SIZES_CONSTANT__ if DWARF register size is constant

And use that to speed up the libgcc unwinder.

gcc/

	* debug.h (dwarf_reg_sizes_constant): Declare.
	* dwarf2cfi.cc (dwarf_reg_sizes_constant): New function.

gcc/c-family/

	* c-cppbuiltin.cc (__LIBGCC_DWARF_REG_SIZES_CONSTANT__):
	Define if constant is known.

libgcc/

	* unwind-dw2.c (dwarf_reg_size): New function.
	(_Unwind_GetGR, _Unwind_SetGR, _Unwind_SetGRPtr)
	(_Unwind_SetSpColumn, uw_install_context_1): Use it.
	(uw_init_context_1): Do not initialize dwarf_reg_size_table
	if not in use.