MIPS64: Improve method entry/exit code

Improvements:
- the stack frame is (de)allocated in one step instead of two
- the return address register, RA, is restored early for better
  instruction scheduling
- eliminate unused delay slot

Test: test-art-host-gtest
Test: booted MIPS64 (with 2nd arch MIPS32R2) in QEMU

Change-Id: I55172bd167ed1baced82bc1d542213b93b13c2ce