RFR: JDK-8259656: fixpath.sh changes broke _NT_SYMBOL_PATH in RunTests.gmk

[Erik Joelsson]

Adding FIXPATH_BASE in RunTestsPrebuilt.gmk as that's needed for the FixPath make macro to work.

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Commit messages:
 - Set FIXPATH_BASE in RunTestsPrebuilt.gmk

Changes: https://git.openjdk.java.net/jdk/pull/2460/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2460&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259656
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2460.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2460/head:pull/2460

PR: https://git.openjdk.java.net/jdk/pull/2460