RFR: JDK-8200609 Proper fix for mapfile removal for libjsig

[Magnus Ihse Bursie]

When I removed (mostly) all mapfiles for the JDK libraries, building of 
libjsig started failing on Solaris, and that part was backed out.

Here's a new, and improved solution to remove the mapfile for libjsig, 
even on Solaris.

Bug: https://bugs.openjdk.java.net/browse/JDK-8200609
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8200609-remove-mapfile-for-libjsig/webrev.01

/Magnus