libgcc: Specialize execute_cfa_program in DWARF unwinder for alignments [redo]

libgcc: Specialize execute_cfa_program in DWARF unwinder for alignments [redo]

The parameters fs->data_align and fs->code_align always have fixed
values for a particular target in GCC-generated code.  Specialize
execute_cfa_program for these values, to avoid multiplications.

gcc/c-family/

	* c-cppbuiltin.cc (c_cpp_builtins): Define
	__LIBGCC_DWARF_CIE_DATA_ALIGNMENT__.

libgcc/

	* unwind-dw2-execute_cfa.h: New file.  Extracted from
	the execute_cfa_program function in unwind-dw2.c.
	* unwind-dw2.c (execute_cfa_program_generic): New function.
	(execute_cfa_program_specialized): Likewise.
	(execute_cfa_program): Call execute_cfa_program_specialized
	or execute_cfa_program_generic, as appropriate.