meson: add -mstackrealign on i386 with musl and libucontext

On i386 with musl and libucontext, many tests crash with SIGSEGV
when using fibers implemented on top of ucontext APIs.

For example:
```
 929/1498 libsystemd - systemd:test-event-future                                   FAIL             0.97s   killed by signal 11 SIGSEGV
/* test_sd_event_run_timer */
run-suspend-timer: Scheduling fiber
/home/pmos/build/src/systemd-261-rc3/tools/test-crash-trace.sh: line 18: 19994 Segmentation fault         (core dumped) "$@"
===== exit 139 — replaying under gdb =====
/* test_sd_event_run_timer */
run-suspend-timer: Scheduling fiber

Program received signal SIGSEGV, Segmentation fault.
0xf7d68be3 in sd_event_add_time_relative (e=0xf7ffd3f0, ret=0xf7b01fa0, clock=1, usec=10000, accuracy=0, callback=0x565564ad <inner_timer_handler>, userdata=0xf7b01fa4) at ../src/libsystemd/sd-event/sd-event.c:1455
1455	                void *userdata) {

Thread 1 (process 20485 "test-event-futu"):
 #0  0xf7d68be3 in sd_event_add_time_relative (e=0xf7ffd3f0, ret=0xf7b01fa0, clock=1, usec=10000, accuracy=0, callback=0x565564ad <inner_timer_handler>, userdata=0xf7b01fa4) at ../src/libsystemd/sd-event/sd-event.c:1455
        t = 17868423377094431728
        r = <optimized out>
 #1  0x565574c7 in sd_event_run_timer_fiber (userdata=0x0) at ../src/libsystemd/sd-event/test-event-future.c:329
        inner = 0xf7ffd3f0
        source = 0x0
        counter = 0
        r = <optimized out>
 #2  0xf7dab308 in fiber_entry_point () at ../src/libsystemd/sd-future/fiber.c:208
        _cleanup_log_unset_prefix_5 = 0x0
        __unique_prefix_c6 = 0x5655bb30
        f = 0xf7b06840
        __func__ = "fiber_entry_point"
        fake_stack_save = <optimized out>
 #3  0xf7b032ae in setcontext () from /lib/libucontext.so.1
 No symbol table info available.
 #4  0x00000000 in ?? ()
 No symbol table info available.
```

Building systemd with -mstackrealign makes all observed failures
disappear, suggesting a stack alignment issue in the interaction
between compiler-generated code and the ucontext implementation.

This resembles historical stack alignment issues in glibc's i386
makecontext() implementation, which required fixes to preserve the
expected stack alignment.

As a workaround, add -mstackrealign when building on i386 with musl
and libucontext.

Suggested-by: ChatGPT (OpenAI)

meson.build

@@ -510,6 +510,11 @@ if get_option('mode') == 'developer'
         possible_cc_flags += '-fno-omit-frame-pointer'
 endif
 
+# Work around stack alignment issues observed with musl + libucontext on i386.
+if get_option('libc') == 'musl' and host_machine.cpu_family() == 'x86'
+        possible_cc_flags += '-mstackrealign'
+endif
+
 add_project_arguments(
         cc.get_supported_arguments(
                 basic_disabled_warnings,