6c: automatically #define _64BIT.

This makes it much easier to use a tool like Swig which needs
to run either 8c or 6c on generated code which #include's
"runtime.h".

R=ken2, rsc
CC=golang-dev
https://golang.org/cl/3205041

src/cmd/6c/txt.c

@@ -38,6 +38,7 @@ ginit(void)
 
 	thechar = '6';
 	thestring = "amd64";
+	dodefine("_64BIT");
 	listinit();
 	nstring = 0;
 	mnstring = 0;