Added sys/ucontext.h and sys/user.h to all API levels.

Also
1. Header singal.h now includes <sys/ucontext.h>
2. asm/user.h include sys/user.h
3. change use of user_i387_struct to user_fpregs_struct

See bionic 26a8eb50a84e131d34d10d5d167d67e9995399bd

Change-Id: I9592e98b90d773ef5babb428a6a6ef208fe86327

ndk/platforms/android-8/include/signal.h

@@ -39,6 +39,9 @@
 #include <asm/siginfo.h>
 #undef __ARCH_SI_UID_T
 
+#include <sys/ucontext.h>
+#define __BIONIC_HAVE_UCONTEXT_T
+
 __BEGIN_DECLS
 
 typedef int sig_atomic_t;