feat: allow skip the dependency on _putchar

[neko-para]

Currently, it is required to implement _putchar even don't ever plan using it. This PR adds a config option to skip the calling on it.

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With the options -dead_strip on MacOS or -Wl,-gc-sections on Linux, the symbol does being stripped out thus no need to implement it. But these options have their own side effects, which won't always fit. (e.g. -Wl,-gc-sections would break _init)

[eyalroz]

@neko-para : This repository is abandoned. Please have a look at the active fork.