Make de-duplication of abi elements concurrent.

Bug: 63590537

Test: compared the linked abi dump for libbcinfo without this change and
with this change using header-abi-diff, found no differences.

Test:
Without this change - linking libbcinfo.so.lsdump measured with 'time' :
real	0m37.245s
user	0m36.448s
sys	0m0.796s

With this change - linking libbcinfo.so.lsdump measured with 'time':
real	0m8.618s
user	0m36.240s
sys	0m2.988s

Test: tests/test.py; all tests pass.

Change-Id: Ice08147496b22c232102f1d1c2e41fc3067fbeaf