tccpp.c: correct # stringification

tccpp.c: correct # stringification

Fix handling of escape characters, spaces, and line feeds in macros or
macro arguments that might yet be subject to # stringification.

Should this be an -f option? I think memory usage increases only very
slightly (in particular, while line feeds, stray \s, and spaces are
preserved, comments are not), so it's probably not worth it to make it
one.

Note that macro_subst now checks for stray \s which are still left in
the input stream after macro substitution, if desired.

This patch depends on the previous patch, so if you revert that, please
revert this patch, too.

See http://lists.nongnu.org/archive/html/tinycc-devel/2015-05/msg00002.html