Add @file command line support to gcc/g++ (ported from GCC 4.1)

[argasek]

This patch adds @file command line option support to GCC (see: https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Overall-Options.html#Overall-Options). It's necessary to in order to make the toolchain to work with CLion (otherwise indexing and compilation in IDE doesn't work correctly).

[cahirwpz]

I'll accept this patch after a rework. Please split it into following parts:

1. minimal changes that are required for your main patch to work
2. @file support.

First patch needs to be generated by means of scripts/mk-diff-tree.sh. After successful compilation please copy original gcc's sources to sources/gcc-2.95.3.orig name. Then under sources directory run ../scripts/mk-diff-tree.sh gcc-2.95.3 to update files under patches/gcc-2.95.3 directory.

Second patch should be a standalone file - let's say zzb_atfile_support.diff.

If first patch doesn't break Linux / Cygwin build I'll import second one.

I'd appreciate if you added a short documentation (github wiki) on how to actually configure CLion to make use of the toolchain. That could be beneficial to the community.

[cahirwpz]

Could you move the patch to https://github.com/cahirwpz/amigaos-gcc-2.95.3 ?