Fix the definition of mc_fpstate on FreeBSD x86

The definition added in b811b70 was
technically wrong even though the type size was correct.  It was
probably defined this way because earlier versions of Rust had
difficulty with fixed-size arrays of size greater than 32.

This change is necessary for CI to pass on x86 FreeBSD.

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

(backport <#3948>)
(cherry picked from commit ad2d864)

Author: asomers

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

@@ -32,7 +32,7 @@ s_no_extra_traits! {
         pub mc_fpformat: ::c_int,
         pub mc_ownedfp: ::c_int,
         pub mc_flags: register_t,
-        pub mc_fpstate: [[::c_int; 32]; 4],
+        pub mc_fpstate: [::c_int; 128],
         pub mc_fsbase: register_t,
         pub mc_gsbase: register_t,
         pub mc_xfpustate: register_t,