Fix the build on armv7-unknown-freebsd

PR rust-lang#3848 broke the build on armv7-unknown-freebsd by defining a field to
be of an unknown type.  Use the correct type name `usize` instead of
`::__size_t`.

(backport <rust-lang#3994>)
(cherry picked from commit 816b524)

Author: asomers

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

@@ -12,7 +12,7 @@ pub type __gregset_t = [::__greg_t; 17];
 s_no_extra_traits! {
     pub struct mcontext_t {
         pub __gregs: ::__gregset_t,
-        pub mc_vfp_size: ::__size_t,
+        pub mc_vfp_size: usize,
         pub mc_vfp_ptr: *mut ::c_void,
         pub mc_spare: [::c_uint; 33],
     }