I need to bump the library version again, to fix PR 91067 (again).
After doing that, the make check-abi test fails, because the
GLIBCXX_3.4.27 symbols aren't in the baseline files yet. This updates
the baselines to match what's in gcc-9.2 and trunk already. I'll
commit this to trunk and gcc-9-branch.
Then after that, I'll export the missing symbols and bump the library
version (on trunk and gcc-9-branch).