RFR: JDK-8282532 Add option to explicitly set build platform and remove support for legacy cross compilation flags

[TheShermanTanker]

Currently the only other option for manually configuring the build platform while cross compiling are devkits, which don't work on certain systems and are also more focused on differentiating the build and target compilers instead. This patch adds the ability to explicitly set the build platform through a new option, which can be especially helpful for when autodetection fails and devkits cannot be relied on, and also for simpler cross compilation cases (Like the one described in building.md)

This also removes support for the legacy cross compilation flags as well.

WIP: Translation from markdown to html

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Commit messages:
 - Make error message clearer
 - Allow explicit setting of the build platform when cross compiling

Changes: https://git.openjdk.java.net/jdk/pull/7656/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7656&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282532
  Stats: 40 lines in 2 files changed: 21 ins; 3 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7656.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7656/head:pull/7656

PR: https://git.openjdk.java.net/jdk/pull/7656