RFR: JDK-8027627 LOG=trace hardcodes value to bash

[Magnus Ihse Bursie]

This is an old but trivial bug that I want to fix.

Dmitry Samersoff <dmitry.samersoff at oracle.com> discovered that LOG=trace 
only works if bash is in /bin. He also suggested this patch.

Bug: https://bugs.openjdk.java.net/browse/JDK-8027627
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8027627-dont-hardcode-bash/webrev.01

/Magnus