target/hppa: Add BTLB support to hppa TLB functions

Change the TLB code to store the Block-TLBs at the beginning
of the TLB table. New 4k TLB entries which are added later
shall not overwrite any of the BTLB entries.

Make sure that when the TLB is cleared by the OS via the ptlbe
instruction, the Block-TLBs will not be dropped.

Signed-off-by: Helge Deller <deller@gmx.de>