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/include/program_loading.h

@@ -41,6 +41,9 @@ enum prog_type {
  * set on the last segment loaded. */
 void arch_segment_loaded(uintptr_t start, size_t size, int flags);
 
+/* Return true if arch supports bounce buffer.  */
+int arch_supports_bounce_buffer(void);
+
 /* Representation of a program. */
 struct prog {
 	/* The region_device is the source of program content to load. After