[libgfortran, patch] Silence a warning in fallback round() implementation

[FX]

PR 47970 is about a warning in libgfortran/intrinsics/c99_functions.c for platforms without round() and ceill(), because round() is used in roundl() before it's prototype is given. Moving the round() prototype and implementation earlier in the file fixes it.

Patch tested by manually editing my config.h on x86_64-linux to simulate the conditions. OK to commit to trunk?
FX

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