POSIX says <signal.h> gets you ucontext_t.

POSIX also says that ucontext_t's uc_sigmask has type sigset_t.

MIPS64 strace needs this.

The #define is to keep chromium off our lawn; otherwise it tries to redefine
all this stuff itself. We should probably clean that up and remove the #define.

Change-Id: I765d6a765a33dca7db33cd8c40f408dc98e5c95b

libc/include/signal.h

@@ -48,6 +48,9 @@
 #  include <linux/signal.h>
 #endif
 
+#include <sys/ucontext.h>
+#define __BIONIC_HAVE_UCONTEXT_T
+
 __BEGIN_DECLS
 
 typedef int sig_atomic_t;