RFR: JDK-8241271 Make hotspot build reproducible

Thu Mar 19 17:20:41 UTC 2020

Currently, it is not possible two make hotspot builds reproducible, 
since the current date and time is always inserted using the special 
macros __DATE__ and __TIME__. This patch makes it possible to override 
these values from the build system, paving the way for making re-builds 
of hotspot bit-by-bit identical.

On linux/gcc, we can now actually get bit-by-bit identical builds of 
libjvm.so (if not using precompiled headers), so I've updated the 
baseline-cmp profiles and compare script to reflect this.

Bug: https://bugs.openjdk.java.net/browse/JDK-8241271
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8241271-make-hotspot-reproducible/webrev.01

/Magnus