RFR: 8277762: Allow configuration of HOTSPOT_BUILD_USER

[Andrew Leonard]

To better allow "reproducible builds", a new configure parameter is added to set the USERNAME build variable, rather than always using the current user:
    --with-build-user=<username> 
HOTSPOT_BUILD_USER is then set to a reproducible USERNAME if required.

Signed-off-by: Andrew Leonard <anleonar at redhat.com>

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Commit messages:
 - 8277762: Allow configuration of HOTSPOT_BUILD_USER
 - 8277762: Allow configuration of HOTSPOT_BUILD_USER
 - 8277762: Allow configuration of HOTSPOT_BUILD_USER

Changes: https://git.openjdk.java.net/jdk/pull/6542/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6542&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277762
  Stats: 9 lines in 1 file changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6542.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6542/head:pull/6542

PR: https://git.openjdk.java.net/jdk/pull/6542