Merge abi diffs, improve linking performance.

Bug: 38325544

Test:
As shown by out/build.trace.gz when using built header-abi-linker to
link .sdump files produced by header-abi-linker.

Without fix:
Time taken to produce libc.so.lsdump by header-abi-linker => ~51 sec
With fix:
Time taken to produce libc.so.lsdump by header-abi-linker => ~3 sec

Bug: 38325929

Test: merged abiff reports of libjpeg, libc.

Change-Id: Iae54f4754d8b0407fabdb8bc8550a7f7b3479f8f