linux-user/syscall.c: add target-to-host mapping for epoll_create1()

Noticed by Barnabás Virágh as a python-3.7 failue on qemu-alpha.

The bug shows up on alpha as it's one of the targets where
EPOLL_CLOEXEC differs from other targets:
    sysdeps/unix/sysv/linux/alpha/bits/epoll.h: EPOLL_CLOEXEC  = 01000000
    sysdeps/unix/sysv/linux/bits/epoll.h:        EPOLL_CLOEXEC = 02000000

Bug: https://bugs.gentoo.org/717548
Reported-by: Barnabás Virágh
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
CC: Riku Voipio <riku.voipio@iki.fi>
CC: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200415220508.5044-1-slyfox@gentoo.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/syscall.c

@@ -12012,7 +12012,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
 #endif
 #if defined(TARGET_NR_epoll_create1) && defined(CONFIG_EPOLL_CREATE1)
     case TARGET_NR_epoll_create1:
-        return get_errno(epoll_create1(arg1));
+        return get_errno(epoll_create1(target_to_host_bitmask(arg1, fcntl_flags_tbl)));
 #endif
 #if defined(TARGET_NR_epoll_ctl)
     case TARGET_NR_epoll_ctl: