* Makefile (check-data): Use $(abi-name) before other guesses.
Look in $(add-ons) dirs before scripts/data/.
* elf/Makefile (check-data): Likewise.
* scripts/soversions.awk: Grok ABI line.
* Makeconfig ($(common-objpfx)soversions.mk): Likewise.
Emit definition for abi-name variable.
if [__USE_BSD].