RFR: JDK-8282532: Allow explicitly setting build platform alongside --openjdk-target [v6]

[Julian Waters]

> 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)
> 
> WIP: Translation from markdown to html

Julian Waters has updated the pull request incrementally with one additional commit since the last revision:

  Make configure warning more accurate

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7656/files
  - new: https://git.openjdk.java.net/jdk/pull/7656/files/c652b2fc..a19ce21b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7656&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7656&range=04-05

  Stats: 7 lines in 1 file changed: 2 ins; 0 del; 5 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