RFR (XS): 8058317: Top-level Makefiles uses deprecated target jvmg in HotSpot Makefiles

[Erik Helin]

Hi all,

the top-level Makefiles are using the target jvm in the HotSpot 
Makefiles when configured with --debug-level=slowdebug. The jvmg target 
has been deprecated in the HotSpot Makefiles, the debug target should be 
used instead.

This small patch changes jdk-options.m4 to use the debug target instead 
of jvmg.

Bug: https://bugs.openjdk.java.net/browse/JDK-8058317

Webrev: http://cr.openjdk.java.net/~ehelin/8058317/webrev.00/

Testing:
- JPRT
- Verified manually on personal machine that the debug target is now
   being used.

Thanks,
Erik