libcpp: deduplicate definition of padding size

Tie together the two functions that ensure tail padding with
search_line_ssse3 via CPP_BUFFER_PADDING macro.

libcpp/ChangeLog:

	* internal.h (CPP_BUFFER_PADDING): New macro; use it ...
	* charset.cc (_cpp_convert_input): ...here, and ...
	* files.cc (read_file_guts): ...here, and ...
	* lex.cc (search_line_ssse3): here.