Patch 5.6, 5.8 and 5.10 to use freestanding preprocessor

This patch adds the use of the `-ffreestanding` preprocessor option when running scripts through Perl's (absent since 5.12) `-P` command line option, allowing the `comp/cpp.t` and `run/switchPx.t` tests to pass.

I've tested compilation without and with this patch on these production release Perls:
* 5.6.0, 5.6.1, 5.6.2
* 5.8.8, 5.8.9
* 5.10.0, 5.10.1

In each case, they fail the preprocessor tests without the patch and pass with the patch applied.

The underlying reason for the failure is the presence of C block comments in common header files (`/usr/include/stdc-predef.h` on my test system). Perl adds the `-C` option to prevent the preprocessor from stripping anything looking like C comments (regular expressions often do) but that means the block comments from the common file appear in the preprocessed Perl source and break parsing...