Handle string or character literals in a skipped #if block properly.

According to the C standard, tokenization takes place before
processing macro directives. That means continuation lines (backslash
before newline), comments, string or character literals, or any other
tokens in a #if block need to be read properly (and then discarded).

8cc didn't handle comments, string literals and character literals
in a skipped #if block properly.
Make get() always return n for a newline.

Previously get() returned '\r' if a newline is either "\r" or "\r\n".
This patch makes get() return '\n' in all cases. This should simplify
the lexer a bit.