[libgfortran, patch] Silence a warning in libgfortran's runtime/error.c

[FX]

This patch for PR 47972 uses __builtin_choose_expr instead of the current 
if-else, avoiding the type warning for the branch not taken. This was suggested 
by Jakub in the PR itself.

Bootstrapped and regtested on x86_64-linux, OK to commit to trunk?
FX

typewarning.ChangeLog
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typewarning.diff
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