Fix MSVC linker error LNK1170 with long rspfiles

The MSVC linker errors out if a line in the rspfile is too long.
The resolution is to use the built-in ninja keyword $in_newline instead of $in, which splits each input into separate lines.

mesonbuild/backend/ninjabackend.py

@@ -237,8 +237,10 @@ class NinjaRule:
         return ninja_quote(qf(str(x)))
 
     def write(self, outfile: T.TextIO) -> None:
+        rspfile_args = self.args
         if self.rspfile_quote_style is RSPFileSyntax.MSVC:
             rspfile_quote_func = cmd_quote
+            rspfile_args = [NinjaCommandArg('$in_newline', arg.quoting) if arg.s == '$in' else arg for arg in rspfile_args]
         else:
             rspfile_quote_func = gcc_rsp_quote
 
@@ -253,7 +255,7 @@ class NinjaRule:
             if rsp == '_RSP':
                 outfile.write(' command = {} @$out.rsp\n'.format(' '.join([self._quoter(x) for x in self.command])))
                 outfile.write(' rspfile = $out.rsp\n')
-                outfile.write(' rspfile_content = {}\n'.format(' '.join([self._quoter(x, rspfile_quote_func) for x in self.args])))
+                outfile.write(' rspfile_content = {}\n'.format(' '.join([self._quoter(x, rspfile_quote_func) for x in rspfile_args])))
             else:
                 outfile.write(' command = {}\n'.format(' '.join([self._quoter(x) for x in self.command + self.args])))
             if self.deps: