Always accept .o and .obj files extensions from CLI for all platforms

[denizzzka]

Proposal to discuss
Explanation: ldc-developers/ldc#4733

This patch makes DMD behave the same as clang and maybe all other modern compilers. Seems, there is no point in discarding *.o or *.obj depending on current OS - both extensions point us that this is object file

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#20609"

[rikkimax]

Don't forget to make this change in dub as well, it does filtering.

[denizzzka]

@rikkimax It seems like no changes are needed to DUB: this PR only affects reading of external object files provided by 3rd party build systems (like CMake). If current DUb does things correct it will not need to fix anything

[dkorpel]

The idea looks okay to me, I just have a comment about the code quality.

[thewilsonator]

the wording here could be improved. Will come back to this when I am properly awake.