* scripts/firstversions.awk: Allow multiple version sets in the
"earliest version" specification, meaning that version sets in the
gaps between listed versions should be folded into the earliest later
version that is explicitly listed.
* shlib-versions (mips.*-.*-linux.*): Use that syntax for to say
we support GLIBC_2.0 and GLIBC_2.2 but not the intervening sets.
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.
