Ruby: Output a single binary file per platform-specific gem

[johnnyshields]

Fixes #30933 related to Ruby UCRT build.

Note that this reverts the UCRT CPP flag change in #31051 because it's no longer needed.

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In #30933, I discovered the Ruby UCRT gem attempting to load the MSVCRT binary, and failing because that binary file is a zero-byte placeholder. Specifically, the gem root dir looks like this:

grpc_c.32-msvcrt.ruby - 0 KB
grpc_c.64-msvcrt.ruby - 0 KB
grpc_c.64-ucrt.ruby - 9,650 KB

It would be possible to fix the reference so the UCRT binary is loaded (probably a C build flag issue), however...

Rather than having 3 binaries files (1 real + 2 placeholders), the most elegant solution is to only create 1 binary file for each platform-specific, named grpc_c.ruby, which is:

• On Windows: the correct binary file for the given Windows architecture
• On Linux/Mac: a 0-byte placeholder

For reference, the UCRT gem root dir will now look like this:

grpc_c.ruby - 9,650 KB

This solution unambiguously avoids loading the wrong binary, and also gets rid of a lot of cruft in the build code at the same time. Obviously we'll never need more than one grpc_c binary for any given platform gem.

[linux-foundation-easycla]

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• ✅ login: johnnyshields / name: Johnny Shields (4e35e36)

[johnnyshields]

@apolcyn thanks for the fix in v1.49.1. Can you please still look to merge this PR? I think it will simplify things and prevent issues in the future.

[johnnyshields]

@apolcyn let's merge this one? Just want to be completely wrapped up on my GRPC work 👍

[apolcyn]

Sorry for the delay @johnnyshields. Aiming to go through this this week.

[johnnyshields]

@apolcyn pinging on this one.

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