shared/options: fix crash by aligning struct to sizeof(void*)

On some architectures like m68k, alignof(void*) is 2, not sizeof(void*)
(which is 4). So the natural alignment of struct Option is 2 and
sizeof(Option) == 26.

However, each variable placed in the SYSTEMD_OPTIONS ELF section via
_OPTION() carries _alignptr_ (= __attribute__((aligned(sizeof(void*))))),
which forces each entry to start at a 4-byte boundary. The linker
therefore inserts 2 bytes of padding between adjacent entries, producing
an actual stride of 28 in the section.

option_parse() iterates over the section with pointer arithmetic
("opt++"), which advances by sizeof(Option) == 26 and lands inside the
padding. The fields read back as zero, and since commit cf88903637
("tree-wide: get rid of most uses of ALIGN_PTR") added the ordering
assertion below, the resulting "0 < 0" trips it when running --help:

 Assertion 'opt->id < (opt + 1)->id' failed at src/shared/options.c:116, function option_parse(). Aborting.

 #0  0xc0a7b248 in ?? () from /usr/lib/m68k-linux-gnu/libc.so.6
 #1  0xc0a7b2ce in pthread_kill () from /usr/lib/m68k-linux-gnu/libc.so.6
 #2  0xc0a2edc6 in raise () from /usr/lib/m68k-linux-gnu/libc.so.6
 #3  0xc0a1c128 in abort () from /usr/lib/m68k-linux-gnu/libc.so.6
 #4  0xc05c6f78 in option_parse (options=0x0, options_end=0x0, state=0xc09ca968) at ../src/shared/options.c:116

Fix this by applying _alignptr_ to the struct definition itself, so that
sizeof(Option) is padded up to a multiple of sizeof(void*) and matches
the actual on-disk stride. Add an assert_cc() so any future regression
is caught at compile time.

The same latent bug applies to Verb and TestFunc, which use the same
section-placement pattern. Their natural sizeof was already a multiple
of sizeof(void*) so no crash was observed, but apply the same fix
defensively.

Follow-up for cf88903637

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>

src/shared/verbs.h

@@ -11,7 +11,8 @@ typedef enum VerbFlags {
         VERB_GROUP_MARKER = 1 << 2,  /* Fake verb entry to separate groups */
 } VerbFlags;
 
-typedef struct {
+/* Note: see the comment on struct Option in options.h for why _alignptr_ is required here. */
+typedef struct _alignptr_ {
         const char *verb;
         unsigned min_args, max_args;
         VerbFlags flags;
@@ -20,6 +21,7 @@ typedef struct {
         const char *argspec;
         const char *help;
 } Verb;
+assert_cc(sizeof(Verb) % sizeof(void*) == 0);
 
 #define _VERB_DATA(d, v, a, amin, amax, f, dat, h)                      \
         _section_("SYSTEMD_VERBS")                                      \