Fixing linker warnings with Arm Compiler toolchain (C++)

[MatthiasHertel80]

The Arm Compiler toolchains will issue warnings about multiple definitions of flatbuffer_version_string, despite the weak attribute.
Warning: L6439W: Multiply defined Global Symbol flatbuffers::flatbuffer_version_string defined in .data._ZN11flatbuffers25flatbuffer_version_stringE(op_resolver.o) rejected in favor of Symbol defined in .data._ZN11flatbuffers25flatbuffer_version_stringE(flatbuffer_conversions.o).

This affects some projects, mainly embedded, where this compiler is popular. This can be seen in issues, like:
tensorflow/tensorflow#45686
https://forum.edgeimpulse.com/t/not-compiling-symbol-multiply-defined/496

I recommend to mimic the MSVC builds and not compile in the version string. It will not be used for the documented statistical purpose on those embedded platforms anyways.

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[MatthiasHertel80]

@googlebot I signed it!

[vglavnyy]

Keil (ARMCC) has support of __attribute__((weak)) in GNU mode and the compiler-scpecific keyword __weak.

This change can break user code if someone uses this flatbuffer_version_string in ARMCC GNU mode.
@MatthiasHertel80 Can you add checking for GNU and verify it with the ARMCC compiler?

@aardappel

• The const char *flatbuffer_version_string might be declared as constexpr, probably this would have the same effect as weak in this declaration.
• There is the duplicated method:
  

  flatbuffers/include/flatbuffers/base.h

  Lines 150 to 153 in bd37e67

  	namespace flatbuffers {
  	// Returns version as string "MAJOR.MINOR.REVISION".
  	const char* FLATBUFFERS_VERSION();
  	}

  
  This method requires flatbuffers library and can't be used in header-only mode. It returns a version of flatbuffers.a or flatbuffers.so library. This version might differ from the header one (bad case).

[aardappel]

I don't think there's going to be a ton of people relying on flatbuffer_version_string

In fact, I'd love to remove it all-together, and instead rely on an (inline) function of the same. If you see the comment above it (and similarly at the end of building.md), the intent of my original team where I wrote FlatBuffers was for this string to end up in binaries, and to collect statistics on how many apps use FlatBuffers, to justify further development of it. We're way past that stage now, so this can all be removed.

[dbaileychess]

@MatthiasHertel80 Would you like to make a PR removing this?

[dbaileychess]

This should be closed by #7046. Let me know if it is or not.