[sh4] MMU bug fix

Some bugs on SH4 MMU are fixed.

- When a TLB entry is overwritten or invalidated, tlb_flush_page() should be
  invoked to invalidate old entry.
- When a ASID is changed, tlb_flush() should be invoke to invalidate entries
  which have old ASID.
- The check for shared bit in TLB entry causes multiple TLB hit exception.
  As SH3's MMU, shared bit is ignored.
- ASID is used when MMUCR's SV bit or SR's MD bit is zero.
  No need to check both bits are zero.

(Shin-ichiro KAWASAKI)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5068 c046a42c-6fe2-441c-8c8c-71466251a162
