[flang][runtime] Allow already-documented missing 'w' on edit descriptor (#72901)

As already documented in flang/docs/Extensions.md and implemented in the
compilation-time format validator, we want to support at execution time
the Intel (and presumably also Fujitsu) extension of allowing a missing
width on an edit descriptor in a formatted I/O statement.

Fixes https://github.com/llvm/llvm-project/issues/72597.