Add more tests around JitMemoryRegion.

- Test that dropping the writable mapping and keeping the readable one works.
- Test using MADV_DONTFORK.
- Test behavior of memfd when mapping MAP_PRIVATE.

Also move IsSealFutureWriteSupported to libartbase/mem_fd.

Test: jit_memory_region_test
Change-Id: If6392780595d968588041762a72d7fa46a7b0429
4 files changed