runtime: use alignof to check alignment of ucontext_t

    
    Use alignof rather than assuming a 16 byte alignment.
    
    Reviewed-on: https://go-review.googlesource.com/28913

From-SVN: r240047

gcc/go/gofrontend/MERGE

-b37a9e66ea584885043240f8f6f1d1c0284eadec
+6c1f159cdcb56ebff617f6bbc6c97943a1a8a34d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/proc.c

@@ -166,7 +166,13 @@ static ucontext_t*
 ucontext_arg(void** go_ucontext)
 {
 	uintptr_t p = (uintptr_t)go_ucontext;
-	p = (p + 15) &~ (uintptr_t)0xf;
+	size_t align = __alignof__(ucontext_t);
+	if(align > 16) {
+		// We only ensured space for up to a 16 byte alignment
+		// in libgo/go/runtime/runtime2.go.
+		runtime_throw("required alignment of ucontext_t too large");
+	}
+	p = (p + align - 1) &~ (uintptr_t)(align - 1);
 	return (ucontext_t*)p;
 }