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

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 a6ddf25634 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 a6ddf25634 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.