Add -fkeep-system-includes modifier for -E

This option will cause -E to preserve the #include directives
for system headers, rather than expanding them into the output.
This can greatly reduce the volume of preprocessed source text
in a test case, making test case reduction simpler.

Note that -fkeep-system-includes is not always appropriate. For
example, if the problem you want to reproduce is induced by a
system header file, it's better to expand those headers fully.
If your source defines symbols that influence the content of a
system header (e.g., _POSIX_SOURCE) then -E will eliminate the
definition, potentially changing the meaning of the preprocessed
source. If you use -isystem to point to non-system headers, for
example to suppress warnings in third-party software, those will
not be expanded and might make the preprocessed source less useful
as a test case.