hurd: Add MAP_32BIT and MAP_EXCL

As defined in glibc

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/mach/hurd/bits/mman_ext.h

(backport <rust-lang#4127>)
(cherry picked from commit d078048)

Author: sthibaul

src/unix/hurd/mod.rs

@@ -2734,6 +2734,8 @@ pub const MAP_FIXED: ::c_int = 256;
 pub const MAP_NOEXTEND: ::c_int = 512;
 pub const MAP_HASSEMAPHORE: ::c_int = 1024;
 pub const MAP_INHERIT: ::c_int = 2048;
+pub const MAP_32BIT: ::c_int = 4096;
+pub const MAP_EXCL: ::c_int = 16384;
 pub const MAP_FAILED: *mut ::c_void = !0 as *mut ::c_void;
 pub const MADV_NORMAL: ::c_int = 0;
 pub const MADV_RANDOM: ::c_int = 1;