Pack JIT mini-debug-infos together.

We currently produce one in-memory ELF file per JITed method,
which has significant overheads due to the ELF file headers.

Pack several of those files together regularly to save space.
Similarly, prune information about methods which were GCed.

This reduces the size of JIT mini-debug-info by factor of 10.
The final overhead is 5% to 10% relative to the JIT code size.

Test: ./art/test.py -b -r -t 137
Change-Id: Idfaff8ed9a209e871e815e527f24f36c63a57bbf