RFR: 8256182: Update qemu-debootstrap cross-compilation recipe

[Aleksey Shipilev]

Update the docs with the (f)actual recipe used in JDK-8256127.

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Commit messages:
 - 8256182: Update qemu-debootstrap cross-compilation recipe

Changes: https://git.openjdk.java.net/jdk/pull/1160/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1160&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256182
  Stats: 42 lines in 2 files changed: 32 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1160.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1160/head:pull/1160

PR: https://git.openjdk.java.net/jdk/pull/1160