linux-user: Remove type casts to union type

linux-user: Remove type casts to union type

Casting to a union type is a gcc (and clang) extension. Other compilers
might not support it. This is not a problem today, but the type casts
can be removed easily. Smatch now no longer complains like before:

linux-user/syscall.c:3190:18: warning: cast to non-scalar
linux-user/syscall.c:7348:44: warning: cast to non-scalar

Cc: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>