Fix alignment of mcontext_t on FreeBSD x86

https://github.com/freebsd/freebsd-src/blob/main/sys/x86/include/ucontext.h

(backport <#3948>)
(cherry picked from commit 908fc71)

Author: asomers

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

@@ -7,6 +7,7 @@ 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,