crt: Improve the cpp parameters for preprocessing def files

Use -undef to avoid any compiler specific predefined definitions,
and use -P to skip the line markers - removing the need for the extra
sed command.

The -w command to silence warnings is necessary, since otherwise
there are unnecessary warnings like these:
lib64/msvcrt.def.in:66:37: warning: missing terminating ' character
 ; const  __non_rtti_object::`vftable'

Signed-off-by: Martin Storsjö <martin@martin.st>