linux-user: Enable epoll_pwait syscall for ARM

We have support for the epoll_pwait syscall, but it wasn't enabled for
ARM guests because we hadn't defined the syscall number; correct this
deficiency.

Reported-by: Dave Flogeras <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Signed-off-by: Riku Voipio <[email protected]>

Author: pm215

Diff for: linux-user/arm/syscall_nr.h

@@ -350,7 +350,7 @@
 #define TARGET_NR_vmsplice			(343)
 #define TARGET_NR_move_pages			(344)
 #define TARGET_NR_getcpu			(345)
-					/* 346 for epoll_pwait */
+#define TARGET_NR_epoll_pwait                   (346)
 #define TARGET_NR_kexec_load			(347)
 #define TARGET_NR_utimensat			(348)
 #define TARGET_NR_signalfd			(349)