[XTENSA] Allow debugger to modify the WINDOWBASE register.

For the 'return' command, GDB needs to adjust WINDOWBASE.
In case WB is different from 0, we need to rotate the
window register file and update WINDOWSTART and WMASK.
This patch also removes some ret|= statements for
__get_user/__put_user as the address range was alrady
checked a couple of lines earlier.

Signed-off-by: Chris Zankel <chris@zankel.net>

include/asm-xtensa/elf.h

@@ -82,7 +82,8 @@ typedef struct {
 	elf_greg_t lcount;
 	elf_greg_t sar;
 	elf_greg_t windowstart;
-	elf_greg_t reserved[9+48];
+	elf_greg_t windowbase;
+	elf_greg_t reserved[8+48];
 	elf_greg_t a[64];
 } xtensa_gregset_t;