fix clang warnings

amd64-ucontext.h:3:25: warning: redefinition of typedef 'mcontext_t' is a C11 feature
amd64-ucontext.h:4:25: warning: redefinition of typedef 'ucontext_t' is a C11 feature
context.c:86:3: warning: indirection of non-volatile null pointer will be deleted, not trap

Author: 0intro

context.c

@@ -83,7 +83,7 @@ makecontext(ucontext_t *ucp, void (*func)(void), int argc, ...)
 
 	memset(&ucp->uc_mcontext, 0, sizeof ucp->uc_mcontext);
 	if(argc != 2)
-		*(int*)0 = 0;
+		*(volatile int*)0 = 0;
 	va_start(va, argc);
 	ucp->uc_mcontext.mc_rdi = va_arg(va, int);
 	ucp->uc_mcontext.mc_rsi = va_arg(va, int);

taskimpl.h

@@ -105,19 +105,17 @@ extern	void		makecontext(ucontext_t*, void(*)(), int, ...);
 #endif
 
 #if defined(__linux__) && defined(__amd64__)
-typedef struct mcontext mcontext_t;
-typedef struct ucontext ucontext_t;
+#	if defined(__i386__)
+#		include "386-ucontext.h"
+#	elif defined(__x86_64__)
+#		include "amd64-ucontext.h"
+#	endif
 extern	int		getmcontext(mcontext_t*);
 extern	void		setmcontext(const mcontext_t*);
 #define	setcontext(u)	setmcontext(&(u)->uc_mcontext)
 #define	getcontext(u)	getmcontext(&(u)->uc_mcontext)
 extern	int		swapcontext(ucontext_t*, const ucontext_t*);
 extern	void		makecontext(ucontext_t*, void(*)(), int, ...);
-#	if defined(__i386__)
-#		include "386-ucontext.h"
-#	elif defined(__x86_64__)
-#		include "amd64-ucontext.h"
-#	endif
 #endif
 
 #if defined(__APPLE__)