RFR: 8318692: Add instructions for creating Ubuntu-based sysroot for cross compilation [v3]

[Zixian Cai]

> Sometimes it's useful to have a Ubuntu-based chroot when you want to ensure compatibility of your deployment target and/or have access to packages that might not be in Debian upstream for some architectures.
> 
> Ubuntu-based sysroot warrants a separate instruction because `symlinks` is in the `universe` repository, and without that, `debootstrap` might fail with package not found, but it's not immediately obvious why.
> 
> This should be merged after #16332 for consistent formatting. Once #16332 is merged, I will update this PR.

Zixian Cai has updated the pull request incrementally with two additional commits since the last revision:

 - Update rendered HTML
 - Update building.md

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16331/files
  - new: https://git.openjdk.org/jdk/pull/16331/files/3148ecda..3466a0fc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16331&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16331&range=01-02

  Stats: 12 lines in 2 files changed: 10 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16331.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16331/head:pull/16331

PR: https://git.openjdk.org/jdk/pull/16331