[llvm] [flang-rt][runtime] Optimize formatted writes by reducing ExternalFileUnit::Emit() calls (PR #222522)

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

For completeness, here's related flang Discourse post: https://discourse.llvm.org/t/rfc-reduce-externalfileunit-emit-call-overhead-for-formatted-sequential-output/91674

https://github.com/llvm/llvm-project/pull/222522