libstd should be able to use proc macros

[todo]

Because libstd fails when using proc macro, we had to move swipc-gen to a build.rs script, just like in the good ol' days. Thx I hate it.
We should figure out why proc macros fail when cross-compiling libstd with xargo and fix it.

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SunriseOS/libuser/build.rs

Lines 3 to 13 in 7b7bbcc

	// TODO: libstd should be able to use proc macros
	// BODY: Because libstd fails when using proc macro, we had to move swipc-gen to a build.rs script, just like in the good ol' days. Thx I hate it.
	// BODY:
	// BODY: We should figure out why proc macros fail when cross-compiling libstd with xargo and fix it.
	use std::env;
	use std::fs::{self, File};
	use std::io::Write as _;
	use std::fmt::Write as _;
	use std::path::{Path, PathBuf};

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This issue was generated by todo based on a TODO comment in 7b7bbcc when #492 was merged. cc @Thog.