RFR(S) 8193318: ELF decoder should be able to use external debug info file

[Zhengyu Gu]

This Linux only patch allows OpenJDK Linux distributions, which strip 
out debug symbols in production bundles, to use external debuginfo 
bundles, if present, to decode stacks.

The implementation mirrors SA implementation [1][2]. It has minimal 
impact to Oracle's JDK, which always contains symbol tables.


Bug: https://bugs.openjdk.java.net/browse/JDK-8193318
Webrev: http://cr.openjdk.java.net/~zgu/8193318/webrev.00/

Test:
   hotspot_runtime on Linux x64 (fastdebug and release)

   Manual tests: striping out symbols from libjvm.so and libjli.so to 
force using debuginfo files.

   Submit repo tests came back clean.

[1] https://bugs.openjdk.java.net/browse/JDK-6932270
[2] http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/rev/7de45b5044c3


Thanks,

-Zhengyu