feat: make sysroot a feature

[cloudhan]

Possibly supersede #313.

This should be fully address and tested in the future cross compile support.

[cloudhan]

@lukasoyen I think this will work.

[lukasoyen]

I do like this and would rebase my PR once this is merged. Unfortunately this alone does not fix my issue.

I am having a hard time coming up with a reproducer. But my problem comes from using a rules_cc based toolchain like this one. These new toolchains do not set the builtin_sysroot attribute.

[cloudhan]

I added some flag tests for sysroot. But since it requires us to register a dummy toolchain with sysroot in it, so without a large refactor, I don't we are ready for the merge...

Or maybe use dev_dependency=True and make the toolchain resolution more strict. Not sure if it worth the effort.

[cloudhan]

Due to complexity for test, leave it for future cross compiling.