die() when attempting to use bounce buffer on non-i386.

Only i386 has code to support bounce buffer. For others coreboot
would silently discard part of binary which doesn't work and is a hell to debug.

Instead just die.

Change-Id: I37ae24ea5d13aae95f9856a896700a0408747233
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: https://review.coreboot.org/13750
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/boot.c

@@ -192,6 +192,11 @@ static void jmp_payload(void *entry, unsigned long buffer, unsigned long size)
 		);
 }
 
+int arch_supports_bounce_buffer(void)
+{
+	return 1;
+}
+
 static void try_payload(struct prog *prog)
 {
 	if (prog_type(prog) == PROG_PAYLOAD) {