[llvm] [Flang-rt][build] fixes building warnings under gcc: (PR #173955)

[liao jun via llvm-commits]

zjlcd wrote:

Hi, @eugeneepshteyn , my ubuntu-x86_64 machine and gcc version information: 

<img width="846" height="336" alt="image" src="https://github.com/user-attachments/assets/334413e3-7484-4bae-955d-d400f0d0e5c0" />


some exact example of building warnings:
1. flang-rt/runtime/numeric-templates.h

<img width="1857" height="510" alt="image" src="https://github.com/user-attachments/assets/54ecfbec-22be-481a-bf20-1926c3dc8599" />


2. flang-rt/lib/runtime/character.cpp
<img width="1857" height="511" alt="image" src="https://github.com/user-attachments/assets/fb0a8ad3-7cf9-4428-b0fd-338d8fd7dccf" />

3. flang-rt/lib/runtime/extensions.cpp
<img width="1788" height="139" alt="image" src="https://github.com/user-attachments/assets/6c93a6c7-5c79-4e18-9bd2-1464554532a6" />

All information is provided in the form of images. Thank you!

https://github.com/llvm/llvm-project/pull/173955