supported version for libc 0.2.90.libio to GLIBC_2.2.
* Makeconfig (soversions.mk): Grok new third column in shlib-versions,
and use it to emit new variable `map-firstversions'.
* scripts/firstversions.awk: New file.
* Makerules (Versions.all): Use scripts/firstversions.awk and
the $(map-firstversions) value to generate a modified versions list
that includes renames in "A = B" syntax for each version set earlier
than the "earliest symbol version" named in shlib-versions.
* scripts/versions.awk: Recognize "A = B" lines in the input to mean
rename version set A to B in the output to the intermediate file.
* scripts/abi-versions.awk: New file.
* Makerules (abi-versions.h): New target, generated by that script.
[$(versioning) = yes] (before-compile): Prepend abi-versions.h.
* include/shlib-compat.h: New file, uses that generated header.
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