ENH move to glibc 2.17 (aka cos7)

See original GitHub issue

To do:

split out the sysroot in the current ct-ng packages and use CDT sysroot (https://github.com/conda-forge/ctng-compilers-feedstock/pull/23)
remove cos from the sysroot name for binutils (https://github.com/conda-forge/binutils-feedstock/pull/25)
make new binutils not like old compilers (https://github.com/conda-forge/binutils-feedstock/pull/27)
patch the old binutils packages to have them be incompatible with the sysroot packages (https://github.com/conda-forge/conda-forge-repodata-patches-feedstock/pull/49)
fix issue with track features and current_repodata.json (https://github.com/conda/conda-build/pull/3957, https://github.com/conda-forge/linux-sysroot-feedstock/pull/12, https://github.com/conda-forge/linux-sysroot-feedstock/pull/11)
build the cos7 glibc 2.17 CDT
test compiler w/ new cos7 CDT
make sure the proper virtual package constraints are present in the CDT package and exported to the proper runtimes
fix extra DSO errors (https://github.com/conda/conda-build/pull/3969)
merge final PRs for sysroot shims (https://github.com/conda-forge/binutils-feedstock/pull/32, https://github.com/conda-forge/ctng-compilers-feedstock/pull/29)
~mark the latest version of the compilers as broken to move to new sysroot (https://github.com/conda-forge/cf-mark-broken/pull/67)~
ship 9.3 libs for ctng-compilers feedstock based on new sysroot build (https://github.com/conda-forge/ctng-compilers-feedstock/pull/34)
fix issue with virtual package constraints (https://github.com/conda/conda/issues/9896)
Bring all x86_64 cos6 CDTs into conda-forge
Bring all x86_64 cos7 CDTs into conda-forge
After the 2 above, move CDTs into conda symlink
make old compiler activation scripts conflict with new compilers (https://github.com/conda-forge/conda-forge-repodata-patches-feedstock/pull/68)
add sysroot deps to new compiler activation scripts (https://github.com/conda-forge/ctng-compiler-activation-feedstock/pull/24)
make sure to add a sysroot run dep to gcc_impl, gxx_impl and gfortran_impl so that the repo data patches for the CDTs don’t break the new compilers (conda-forge/ctng-compilers-feedstock#32)
~patch conda-build for sysroot distro name (https://github.com/conda/conda-build/pull/3999)~
fix python sysconfig data

stretch goals

maybe build gcc from a new gcc-feedstock instead of ct-ng?

other relevant issues

uninstalling from symlinks in other packages is broken (https://github.com/conda/conda/issues/9810#issuecomment-607941126)

I (@beckermr) am pretty out of my depth here but ready to learn!

old comment from @xhochy

I have packages that require newer glibc features like setns or process_vm_readv. These are not included in the glibc version currently used for building on Linux. Is there a way to use a newer baseline glibc* for a package or impossible until we up the base CentOS version globally?

*I’m aware that the package would run on less systems but that would do it either way.