[jdk8u-dev] RFR: 8139668: Generate README-build.html from markdown

[Andrew John Hughes]

This updates the build documentation so it `README-builds.html` is generated from `README-builds,md`

This is a precursor to [JDK-8176509](https://bugs.openjdk.org/browse/JDK-8176509) which also updates the `README`, simplifying #75 

There are deliberately no content changes here. In fact, JDK 9 changes to the documentation prior to this change are reverted (VS2010->VS2013 bump and other build requirement changes (namely [JDK-8041593](https://bugs.openjdk.org/browse/JDK-8041593) (JDK9 update), [JDK-8062223](https://bugs.openjdk.org/browse/JDK-8062223) (ccache update), [JDK-8076531](https://bugs.openjdk.org/browse/JDK-8076531) (Windows default compiler change) and [JDK-8072023](https://bugs.openjdk.org/browse/JDK-8072023) (make version change)). We can make our own updates once these changes are in.

The updated `README-builds.html` was generated from the new `README-builds.md`. I fixed a bug in `common/bin/update-build-readme.sh` which checks the variable `MARKDOWN` for the location of the processor but then uses a bare `markdown` in the actual invocation.  This script is removed in JDK-8176509 anyway, being replaced with `pandoc`.

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

Commit messages:
 - Backport 17c896827dbf1a54ab6539cc2b506f973dbde246

Changes: https://git.openjdk.org/jdk8u-dev/pull/79/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=79&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8139668
  Stats: 4159 lines in 3 files changed: 1667 ins; 1445 del; 1047 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/79.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/79/head:pull/79

PR: https://git.openjdk.org/jdk8u-dev/pull/79