linux-user: fix gcc-4.9 compiler error on __{get,put]}_user

gcc-4.9 finds unused operand:

linux-user/syscall.c: In function ‘host_to_target_stat64’:
linux-user/qemu.h:301:19: error: right-hand operand of comma expression
has no effect [-Werror=unused-value]
      ((hptr), (x)), 0)

Just removing the rh operand is no good, it will error in later:

linux-user/main.c: In function ‘arm_kernel_cmpxchg64_helper’:
linux-user/qemu.h:330:15: error: void value not ignored as it ought to be
         __ret = __put_user((x), __hptr);    \

Thus, remove setting __ret from __get_user and __put_user, as and
set the right hand operand to (void)0 to make it clear that these
return never nothing.

This commit depends on the signal.c cleanup, to ensure bisectable
version history.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Richard Henderson <rth@twiddle.net>

linux-user/qemu.h

@@ -299,7 +299,7 @@ static inline int access_ok(int type, abi_ulong addr, abi_ulong size)
    __builtin_choose_expr(sizeof(*(hptr)) == 2, stw_##e##_p,             \
    __builtin_choose_expr(sizeof(*(hptr)) == 4, stl_##e##_p,             \
    __builtin_choose_expr(sizeof(*(hptr)) == 8, stq_##e##_p, abort))))   \
-     ((hptr), (x)), 0)
+     ((hptr), (x)), (void)0)
 
 #define __get_user_e(x, hptr, e)                                        \
   ((x) = (typeof(*hptr))(                                               \
@@ -307,7 +307,7 @@ static inline int access_ok(int type, abi_ulong addr, abi_ulong size)
    __builtin_choose_expr(sizeof(*(hptr)) == 2, lduw_##e##_p,            \
    __builtin_choose_expr(sizeof(*(hptr)) == 4, ldl_##e##_p,             \
    __builtin_choose_expr(sizeof(*(hptr)) == 8, ldq_##e##_p, abort))))   \
-     (hptr)), 0)
+     (hptr)), (void)0)
 
 #ifdef TARGET_WORDS_BIGENDIAN
 # define __put_user(x, hptr)  __put_user_e(x, hptr, be)
@@ -326,9 +326,9 @@ static inline int access_ok(int type, abi_ulong addr, abi_ulong size)
 ({									\
     abi_ulong __gaddr = (gaddr);					\
     target_type *__hptr;						\
-    abi_long __ret;							\
+    abi_long __ret = 0;							\
     if ((__hptr = lock_user(VERIFY_WRITE, __gaddr, sizeof(target_type), 0))) { \
-        __ret = __put_user((x), __hptr);				\
+        __put_user((x), __hptr);				\
         unlock_user(__hptr, __gaddr, sizeof(target_type));		\
     } else								\
         __ret = -TARGET_EFAULT;						\
@@ -339,9 +339,9 @@ static inline int access_ok(int type, abi_ulong addr, abi_ulong size)
 ({									\
     abi_ulong __gaddr = (gaddr);					\
     target_type *__hptr;						\
-    abi_long __ret;							\
+    abi_long __ret = 0;							\
     if ((__hptr = lock_user(VERIFY_READ, __gaddr, sizeof(target_type), 1))) { \
-        __ret = __get_user((x), __hptr);				\
+        __get_user((x), __hptr);				\
         unlock_user(__hptr, __gaddr, 0);				\
     } else {								\
         /* avoid warning */						\