Tweak merged .abilist file format to merge duplicate symbols in
different sets into multi-version stanzas with new "| SET regexp..."
header continuation lines.
* scripts/merge-abilist.awk: Rewritten for new format.
* scripts/extract-abilist.awk: Likewise.
stanzas for each version set, the set name listed only once.
* scripts/extract-abilist.awk: New file.
* scripts/merge-abilist.awk: New file.
* Makerules (check-abi-%, update-abi-%): New pattern rules.
(update-abi, check-abi): New targets.
* Makefile (+subdir_targets): Add subdir_{check,update}-abi.
* Makerules (%.symlist): Use LC_ALL=C when running awk script.
