Add ucontext for linux-musl.

src/unix/notbsd/linux/musl/b32/x86.rs

@@ -82,6 +82,19 @@ s! {
         pub f_flags: ::c_ulong,
         pub f_spare: [::c_ulong; 4],
     }
+
+    pub struct mcontext_t {
+        __private: [u32; 22]
+    }
+
+    pub struct ucontext_t {
+        pub uc_flags: ::c_ulong,
+        pub uc_link: *mut ucontext_t,
+        pub uc_stack: ::stack_t,
+        pub uc_mcontext: mcontext_t,
+        pub uc_sigmask: ::sigset_t,
+        __private: [u8; 112],
+    }
 }
 
 pub const O_DIRECT: ::c_int = 0x4000;

src/unix/notbsd/linux/musl/b64/x86_64.rs

@@ -1,3 +1,18 @@
+s! {
+    pub struct mcontext_t {
+        __private: [u64; 32],
+    }
+
+    pub struct ucontext_t {
+        pub uc_flags: ::c_ulong,
+        pub uc_link: *mut ucontext_t,
+        pub uc_stack: ::stack_t,
+        pub uc_mcontext: mcontext_t,
+        pub uc_sigmask: ::sigset_t,
+        __private: [u8; 512],
+    }
+}
+
 pub const SYS_gettid: ::c_long = 186;
 
 pub const SYS_perf_event_open: ::c_long = 298;