Gate FreeBSD mcontext_t under libc_align

Enable this backport by just dropping support for FreeBSD mcontext_t
without `libc_align`, i.e. with Rust < 1.25 (2018). This is sufficiently
niche that it is not worth adding a new `align` module.

Author: tgross35

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

@@ -1664,6 +1664,7 @@ s_no_extra_traits! {
 
     pub struct ucontext_t {
         pub uc_sigmask: ::sigset_t,
+        #[cfg(libc_align)]
         pub uc_mcontext: ::mcontext_t,
         pub uc_link: *mut ::ucontext_t,
         pub uc_stack: ::stack_t,
@@ -2601,7 +2602,8 @@ cfg_if! {
             fn fmt(&self, f: &mut ::fmt::Formatter) -> ::fmt::Result {
                 f.debug_struct("ucontext_t")
                     .field("uc_sigmask", &self.uc_sigmask)
-                    .field("uc_mcontext", &self.uc_mcontext)
+                    // FIXME: enable when libc_align is dropped
+                    // .field("uc_mcontext", &self.uc_mcontext)
                     .field("uc_link", &self.uc_link)
                     .field("uc_stack", &self.uc_stack)
                     .field("uc_flags", &self.uc_flags)

src/unix/bsd/freebsdlike/freebsd/x86.rs

@@ -47,7 +47,7 @@ s_no_extra_traits! {
 pub(crate) const _ALIGNBYTES: usize = mem::size_of::<c_long>() - 1;
 
 cfg_if! {
-    if #[cfg(feature = "extra_traits")] {
+    if #[cfg(all(libc_align, feature = "extra_traits"))] {
         impl PartialEq for mcontext_t {
             fn eq(&self, other: &mcontext_t) -> bool {
                 self.mc_onstack == other.mc_onstack