RFR: JDK-8213005: Missing symbols in hs_err files on Windows after JDK-8212028

[Erik Joelsson]

When running tests through the new RunTest makefile framework in Mach5, 
we are now missing symbol information in hs_err files on Windows. This 
is caused by the new run-test-prebuilt target not properly setting up 
_NT_SYMBOL_PATH which in turn is caused by the variable CYGPATH not 
being set.

This patch sets CYGPATH in RunTestPrebuiltSpec.gmk. I also modified some 
cosmetics in the _NT_SYMBOLS_PATH setup to make it easier to understand, 
at least for me.

Verfied by running "run-test-prebuilt" with a fastdebug build and 
setting JTREG=JAVA_OPTIONS=-XX:ErrorHandlerTest=14. With the fix, the 
generated hs_err*.log contains symbols in the stacktrace.

Bug: https://bugs.openjdk.java.net/browse/JDK-8213005

Webrev: 
http://cr.openjdk.java.net/~erikj/8213005/webrev.01/make/RunTests.gmk.sdiff.html

/Erik