linux-user: Fix definition of target_sigevent for 32-bit guests

The sigevent structure includes a union with some fields which
are pointers. For the QEMU target_sigevent structure we must
represent these as abi_ulongs, not host function pointers.

This error was causing the compiler to believe it should 8-align
the _sigev_un union on a 64-bit host, which meant that the
code in target_to_host_sigevent() was looking at the wrong
offset to find the _tid field, and timer_create() would
spuriously fail with EINVAL.

This fixes the final loose end noted in LP:1042388.

While we're editing the structure, switch the 'int32_t' fields
to 'abi_int'; this will only matter for guests with non-standard
integer alignment like m68k.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/syscall_defs.h

@@ -2658,15 +2658,19 @@ typedef int32_t target_timer_t;
 
 struct target_sigevent {
     target_sigval_t sigev_value;
-    int32_t sigev_signo;
-    int32_t sigev_notify;
+    abi_int sigev_signo;
+    abi_int sigev_notify;
     union {
-        int32_t _pad[TARGET_SIGEV_PAD_SIZE];
-        int32_t _tid;
+        abi_int _pad[TARGET_SIGEV_PAD_SIZE];
+        abi_int _tid;
 
+        /* The kernel (and thus QEMU) never looks at these;
+         * they're only used as part of the ABI between a
+         * userspace program and libc.
+         */
         struct {
-            void (*_function)(sigval_t);
-            void *_attribute;
+            abi_ulong _function;
+            abi_ulong _attribute;
         } _sigev_thread;
     } _sigev_un;
 };