git: openjdk/jdk: 8318692: Add instructions for creating Ubuntu-based sysroot for cross compilation

[Aleksey Shipilev]

Changeset: c3cdfe2a
Author:    Zixian Cai <zcai at openjdk.org>
Committer: Aleksey Shipilev <shade at openjdk.org>
Date:      2023-10-25 08:33:10 +0000
URL:       https://git.openjdk.org/jdk/commit/c3cdfe2a328c59213b614a2b723184582550f8c7

8318692: Add instructions for creating Ubuntu-based sysroot for cross compilation

Reviewed-by: erikj, shade

! doc/building.html
! doc/building.md