add documentation on howto build just rustc without libstd to the build system

[flo-l]

I searched for days until I found this, maybe this helps other poor souls 😂

for reference #33990

[rust-highfive]

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[flo-l]

cc @alexcrichton

[alexcrichton]

We may want to replace the "x86_64-unknown-linux-gnu" here with "$host" perhaps as it'll differ depending on where you're compiling. The explanation may also want to be accompanied with a warning indicating that this may not work as the standard library really does need to be recompiled to be guaranteed to work.

[flo-l]

good catch with x86_64-unknown-linux-gnu, fixed it and added some more words of warning regarding libstd.

[alexcrichton]

One more "$host" here

[flo-l]

fixed again!

[alexcrichton]

@bors: r+ 4e87986

[eddyb]

@bors rollup

[bors]

⌛ Testing commit 4e87986 with merge 1ef5f79...

[bors]

⛄ The build was interrupted to prioritize another pull request.

[bors]

⌛ Testing commit 4e87986 with merge 71efa91...

[bors]

⛄ The build was interrupted to prioritize another pull request.

[bors]

⌛ Testing commit 4e87986 with merge 3bedec0...

[bors]

⛄ The build was interrupted to prioritize another pull request.