Fix redox MAP_FIXED

(backport <rust-lang#4684>)
(cherry picked from commit 60286e6)

Author: willnode

src/unix/redox/mod.rs

@@ -761,7 +761,7 @@ pub const MAP_SHARED: c_int = 0x0001;
 pub const MAP_PRIVATE: c_int = 0x0002;
 pub const MAP_ANON: c_int = 0x0020;
 pub const MAP_ANONYMOUS: c_int = MAP_ANON;
-pub const MAP_FIXED: c_int = 0x0010;
+pub const MAP_FIXED: c_int = 0x0004;
 pub const MAP_FAILED: *mut c_void = !0 as _;
 
 pub const MS_ASYNC: c_int = 0x0001;