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.

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

@@ -1657,6 +1657,7 @@ 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,
@@ -2670,7 +2671,8 @@ 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

@@ -6,39 +6,43 @@ pub type time_t = i32;
 pub type suseconds_t = i32;
 pub type register_t = i32;
 
-s_no_extra_traits! {
-    #[repr(align(16))]
-    pub struct mcontext_t {
-        pub mc_onstack: register_t,
-        pub mc_gs: register_t,
-        pub mc_fs: register_t,
-        pub mc_es: register_t,
-        pub mc_ds: register_t,
-        pub mc_edi: register_t,
-        pub mc_esi: register_t,
-        pub mc_ebp: register_t,
-        pub mc_isp: register_t,
-        pub mc_ebx: register_t,
-        pub mc_edx: register_t,
-        pub mc_ecx: register_t,
-        pub mc_eax: register_t,
-        pub mc_trapno: register_t,
-        pub mc_err: register_t,
-        pub mc_eip: register_t,
-        pub mc_cs: register_t,
-        pub mc_eflags: register_t,
-        pub mc_esp: register_t,
-        pub mc_ss: register_t,
-        pub mc_len: ::c_int,
-        pub mc_fpformat: ::c_int,
-        pub mc_ownedfp: ::c_int,
-        pub mc_flags: register_t,
-        pub mc_fpstate: [::c_int; 128],
-        pub mc_fsbase: register_t,
-        pub mc_gsbase: register_t,
-        pub mc_xfpustate: register_t,
-        pub mc_xfpustate_len: register_t,
-        pub mc_spare2: [::c_int; 4],
+cfg_if! {
+    if #[cfg(libc_align)] {
+        s_no_extra_traits! {
+            #[repr(align(16))]
+            pub struct mcontext_t {
+                pub mc_onstack: register_t,
+                pub mc_gs: register_t,
+                pub mc_fs: register_t,
+                pub mc_es: register_t,
+                pub mc_ds: register_t,
+                pub mc_edi: register_t,
+                pub mc_esi: register_t,
+                pub mc_ebp: register_t,
+                pub mc_isp: register_t,
+                pub mc_ebx: register_t,
+                pub mc_edx: register_t,
+                pub mc_ecx: register_t,
+                pub mc_eax: register_t,
+                pub mc_trapno: register_t,
+                pub mc_err: register_t,
+                pub mc_eip: register_t,
+                pub mc_cs: register_t,
+                pub mc_eflags: register_t,
+                pub mc_esp: register_t,
+                pub mc_ss: register_t,
+                pub mc_len: ::c_int,
+                pub mc_fpformat: ::c_int,
+                pub mc_ownedfp: ::c_int,
+                pub mc_flags: register_t,
+                pub mc_fpstate: [::c_int; 128],
+                pub mc_fsbase: register_t,
+                pub mc_gsbase: register_t,
+                pub mc_xfpustate: register_t,
+                pub mc_xfpustate_len: register_t,
+                pub mc_spare2: [::c_int; 4],
+            }
+        }
     }
 }
 
@@ -54,7 +58,7 @@ cfg_if! {
 }
 
 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 &&