linux-user: xtensa: enable bFLT support

- request bflt support in configure;
- implement custom linux-user/xtensa/target_flat.h that doesn't put envp
  on stack;
- fix #include "target_flat.h" in flatload.c so that it first search for
  arch-customized version of the header.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

configure

@@ -7178,6 +7178,7 @@ case "$target_name" in
   ;;
   xtensa|xtensaeb)
     TARGET_ARCH=xtensa
+    bflt="yes"
     mttcg="yes"
     target_compiler=$cross_cc_xtensa
   ;;

linux-user/flatload.c

@@ -37,7 +37,7 @@
 
 #include "qemu.h"
 #include "flat.h"
-#include "target_flat.h"
+#include <target_flat.h>
 
 //#define DEBUG
 

linux-user/xtensa/target_flat.h

@@ -0,0 +1,10 @@
+/* If your arch needs to do custom stuff, create your own target_flat.h
+ * header file in linux-user/<your arch>/
+ */
+#define flat_argvp_envp_on_stack()                           0
+#define flat_reloc_valid(reloc, size)                        ((reloc) <= (size))
+#define flat_old_ram_flag(flag)                              (flag)
+#define flat_get_relocate_addr(relval)                       (relval)
+#define flat_get_addr_from_rp(rp, relval, flags, persistent) (rp)
+#define flat_set_persistent(relval, persistent)              (*persistent)
+#define flat_put_addr_at_rp(rp, addr, relval)                put_user_ual(addr, rp)