Modified the floppy boot image target so that it can be used to build an x86_64 boot image.

* set_haiku_revision doesn't currently support ELF64, don't use a
  revisioned kernel image on x86_64 for now.
* Don't try to build add-ons for x86_64 yet.

build/jam/FloppyBootImage

@@ -52,6 +52,7 @@ SYSTEM_ADD_ONS_BUS_MANAGERS = $(X86_ONLY)acpi $(ATA_ONLY)ata config_manager dpc
 SYSTEM_ADD_ONS_FILE_SYSTEMS = bfs iso9660 attribute_overlay write_overlay ;
 
 
+if $(TARGET_ARCH) != x86_64 {
 	# modules
 	AddFilesToFloppyBootArchive system add-ons kernel bus_managers
 		: $(SYSTEM_ADD_ONS_BUS_MANAGERS) ;
@@ -87,9 +88,15 @@ if $(NET_BOOT) = 1 {
 		AddDriversToFloppyBootArchive disk virtual : remote_disk ;
 		AddDriversToFloppyBootArchive net : $(SYSTEM_ADD_ONS_DRIVERS_NET) ;
 	}
+}
 
 # kernel
+if $(TARGET_ARCH) = x86_64 {
+	# TODO x86_64: ELF64 support not added to set_haiku_revision.
+	AddFilesToFloppyBootArchive system : kernel_$(TARGET_ARCH) ;
+} else {
 	AddFilesToFloppyBootArchive system : <revisioned>kernel_$(TARGET_ARCH) ;
+}
 
 # scripts and data files
 
@@ -114,6 +121,7 @@ if $(NET_BOOT) = 1 {
 }
 
 # boot module links
+if $(TARGET_ARCH) != x86_64 {
 	AddBootModuleSymlinksToFloppyBootArchive
 		$(SYSTEM_ADD_ONS_BUS_MANAGERS)
 		$(PPC_ONLY)openpic
@@ -127,6 +135,7 @@ AddBootModuleSymlinksToFloppyBootArchive
 		$(SYSTEM_ADD_ONS_FILE_SYSTEMS)
 		$(BOOT_ADD_ONS_NET)
 	;
+}
 
 
 #pragma mark - Build The Archive