[jdk11u-dev] RFR: 8265343: Update Debian-based cross-compilation recipes

[Aleksey Shipilev]

Another (hopefully final) backport to improve cross-compilation docs. The simplified cross-compilation recipe matches what we have in 11u GHA: only `--openjdk-target` and `--with-sysroot` is supplied.

It had minor contextual conflicts that precluded the automatic merge. I checked that the platforms listed in compatible platforms list is indeed buildable (see https://builds.shipilev.net/openjdk-jdk11-dev/ for example).

-------------

Commit messages:
 - Backport 7879adbe88ec54b8f5ac9543d80142bc1b12112b

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/601/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=601&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265343
  Stats: 139 lines in 2 files changed: 86 ins; 15 del; 38 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/601.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/601/head:pull/601

PR: https://git.openjdk.java.net/jdk11u-dev/pull/601