For abi diffing, factor in arch variant and primary arch.

binder_size_t has a different size for builds with different primary
arches.

Also maintain seperate reference dumps for different arch variants,
since different cflags may be legally specified for them (similar to
what GSI does)

Test: create reference dump for libjpeg at
      prebuilts/abi-dumps/vndk/current/arm64/arm64_armv8-a/source-based/libjpeg.so.lsdump.gz
      mm -j64; header-abi-diff gets invoked.

Change-Id: I55eae4d4811c9754fe8dbd1009c7929fea119eeb