loader: fix userboot's ability to detect a guest's interpreter

Some time after r338418, I believe with -Os/-Oz -ffunction-sections
-fdata-sections, the bootprog_interp variable that held our "$Interpreter:"
marker started getting strip from all loaders, with exception to userboot
since it used bootprog_interp to determine what flavor of userboot it was.

At some point, it had been brought to my attention that this was no longer
working and I had worked up some potential solutions to use the variable
that involved printing it out. My vague recollection is that this was
rejected, and I forgot to explore the alternatives; I cannot find records of
this discussion anymore.

Fast forward to present day, Andrew reported that it was non-functional and
offered (effectively) this patch (sans comment) to stop the compiler from
optimizing it out by assigning it to a volatile variable. This removes
concerns about user-facing change while retaining the interpreter marker.
Furthermore, it could certainly be uglier.

Reported and tested by:	Andrew Gierth <andrew_tao173.riddles.org.uk>
MFC after:	3 days

stand/common/interp.c

@@ -45,8 +45,17 @@ __FBSDID("$FreeBSD$");
 void
 interact(void)
 {
-	static char	input[256];			/* big enough? */
+	static char		input[256];		/* big enough? */
+	const char * volatile	interp_identifier;
 
+	/*
+	 * Because interp_identifier is volatile, it cannot be optimized out by
+	 * the compiler as it's considered an externally observable event.  This
+	 * prevents the compiler from optimizing out our carefully placed
+	 * $Interpreter:4th string that userboot may use to determine that
+	 * we need to switch interpreters.
+	 */
+	interp_identifier = bootprog_interp;
 	interp_init();
 
 	printf("\n");