Add an intptr type to runtime; needed in FreeBSD

In thread.c, we need to cast to whatever the native
size of intptr is on the system, but we only have
uintptr available. They're the same size, but can't
do signed casts without this one :).

R=rsc
CC=golang-dev
https://golang.org/cl/156073

src/pkg/runtime/freebsd/thread.c

@@ -141,7 +141,7 @@ newosproc(M *m, G *g, void *stk, void (*fn)(void))
 	param.arg = m;
 	param.stack_base = stk;
 	param.stack_size = g->stackbase - g->stackguard + 256;
-	param.child_tid = (int32*)&m->procid;
+	param.child_tid = (intptr*)&m->procid;
 	param.parent_tid = nil;
 	param.tls_base = (int8*)&m->tls[0];
 	param.tls_size = sizeof m->tls;

src/pkg/runtime/runtime.h

@@ -18,8 +18,10 @@ typedef	double			float64;
 
 #ifdef _64BIT
 typedef	uint64		uintptr;
+typedef	int64		intptr;
 #else
 typedef	uint32		uintptr;
+typedef int32		intptr;
 #endif
 
 /*