ninjabackend: bring back historical "include_directories(is_system: true)" ordering

[bonzini]

Prior to commit a6ddf2534 ("backend/ninja: Extend IncludeDirs.rel_string_list for backend use"), when include_directories() was called with is_system: true and multiple directories, the resulting -isystem flags appeared on the command line in the opposite order from what was specified: the last-listed directory was searched first by the compiler. This was the opposite convention from non-system -I includes, where the first-listed directory is searched first.

The asymmetry was not intentional; it was a side effect of how CompilerArgs.__iadd__ handles flags that are not in prepend_prefixes:

• -I is in CLikeCompilerArgs.prepend_prefixes, so __iadd__ prepends it. The ninja backend iterates directories in reversed order and adds them one at a time; the prepend-on-each-add counteracts the reversal, producing the original order.

• -isystem was not in prepend_prefixes, so __iadd__ appended it. The same reversed one-at-a-time iteration then produced a reversed sequence — the last-listed directory ended up first on the command line.

Commit a6ddf25 changed generate_inc_dir to add all directories in a single bulk commands += call instead of one at a time. This preserved the correct order for -I (the double reversal in __iadd__ still works for a bulk add) but silently broke -isystem, because appending a list at once preserves input order rather than reversing it.

Undo the changes a6ddf25 to preserve the previous behavior. Adding -isystem to prepend_prefixes would also make the ordering consistent, but projects that relied on the old reversed behaviour would silently break.

[bonzini]

See #15577 for another attempt at a fix that didn't work well. Maybe it could be revisited once the "object model for compiler arguments" idea pans out.