RFC: add `strong_constrains` on `sysroot_{{ target_platform }}`?

In https://github.com/conda-forge/libcxx-feedstock/pull/131, I noticed that setting `c_stdlib_version` to 2.17 (and being in a cos7 image) still leads to
```
$PREFIX/bin/x86_64-conda-linux-gnu-ld: $PREFIX/lib/libc++abi.so: undefined reference to `memcpy@GLIBC_2.14'
$PREFIX/bin/x86_64-conda-linux-gnu-ld: $PREFIX/lib/libc++abi.so: undefined reference to `aligned_alloc@GLIBC_2.16'
```
if `sysroot_linux-64 2.17` is not available at test time.

It seems reasonable to me that when a package indicates it requires `c_stdlib_version >=x`, that we shouldn't allow pulling in older sysroots when building against that package?

conda-build has a similar feature like run-exports also for adding `run_constrained:` entries, which would do the job here IMO.

Thoughts @beckermr @isuruf?