linux-user: Use correct alignment for long long on i386 guests

For i386, the ABI specifies that 'long long' (8 byte values)
need only be 4 aligned, but we were requiring them to be
8-aligned. This meant we were laying out the target_epoll_event
structure wrongly. Add a suitable ifdef to abitypes.h to
specify the i386-specific alignment requirement.
Reported-by: NIcenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

include/exec/user/abitypes.h

@@ -15,6 +15,10 @@
 #define ABI_LLONG_ALIGNMENT 2
 #endif
 
+#if defined(TARGET_I386) && !defined(TARGET_X86_64)
+#define ABI_LLONG_ALIGNMENT 4
+#endif
+
 #ifndef ABI_SHORT_ALIGNMENT
 #define ABI_SHORT_ALIGNMENT 2
 #endif