Add MAP_32BIT to 64-bit FreeBSD platforms

Author: Susurrus

src/unix/bsd/freebsdlike/freebsd/aarch64.rs

@@ -28,3 +28,5 @@ s! {
         pub st_birthtime_nsec: ::c_long,
     }
 }
+
+pub const MAP_32BIT: ::c_int = 0x00080000;

src/unix/bsd/freebsdlike/freebsd/x86_64.rs

@@ -28,3 +28,5 @@ s! {
         pub st_birthtime_nsec: ::c_long,
     }
 }
+
+pub const MAP_32BIT: ::c_int = 0x00080000;