Truncate user and group ids

Calls to `getuid`, `getgid` and their `eid` variants fail to compile on
64bit Linux systems because the return value of the syscall is of
`usize` and needs to be truncated to fit the size of `uid_t` that is 32
bit.

Thanks to @FireFox317 for figuring this out in Zig's Discord channel!

lib/std/os/linux.zig

@@ -747,31 +747,31 @@ pub fn getuid() uid_t {
     if (@hasField(SYS, "getuid32")) {
         return @as(uid_t, syscall0(.getuid32));
     } else {
-        return @as(uid_t, syscall0(.getuid));
+        return @truncate(uid_t, syscall0(.getuid));
     }
 }
 
 pub fn getgid() gid_t {
     if (@hasField(SYS, "getgid32")) {
         return @as(gid_t, syscall0(.getgid32));
     } else {
-        return @as(gid_t, syscall0(.getgid));
+        return @truncate(gid_t, syscall0(.getgid));
     }
 }
 
 pub fn geteuid() uid_t {
     if (@hasField(SYS, "geteuid32")) {
         return @as(uid_t, syscall0(.geteuid32));
     } else {
-        return @as(uid_t, syscall0(.geteuid));
+        return @truncate(uid_t, syscall0(.geteuid));
     }
 }
 
 pub fn getegid() gid_t {
     if (@hasField(SYS, "getegid32")) {
         return @as(gid_t, syscall0(.getegid32));
     } else {
-        return @as(gid_t, syscall0(.getegid));
+        return @truncate(gid_t, syscall0(.getegid));
     }
 }