Make LuaJIT compile on s390x.

I've disabled both the JIT and FFI for now. I've also stripped almost
all of the assembly out of vm_s390x.dasc, leaving only labels for the
most part. This is enough to get LuaJIT to compile but of course if
you try and run it it will explode.

The idea now is to re-add enough functionality to get a very basic
Lua program to run.

src/lib_jit.c

@@ -731,6 +731,8 @@ static uint32_t jit_cpudetect(lua_State *L)
   }
 #endif
 #endif
+#elif LJ_TARGET_S390X
+  /* No optional CPU features to detect (for now). */
 #else
 #error "Missing CPU detection for this architecture"
 #endif

src/lj_arch.h

@@ -370,6 +370,7 @@
 #define LJ_ARCH_NUMMODE		LJ_NUMMODE_SINGLE_DUAL
 #define LJ_TARGET_GC64		1
 #define LJ_ARCH_NOJIT		1	/* NYI */
+#define LJ_ARCH_NOFFI		1	/* Disable FFI for now. */
 
 #else
 #error "No target architecture defined"