RFR: JDK-8180342 fixup path for jtreg

[Magnus Ihse Bursie]

JDK-8180281 caused a regression on certain windows configurations for 
jtreg. We need to clean up the paths.

I have verified that this resolves the issue on the reported configuration.

Bug: https://bugs.openjdk.java.net/browse/JDK-8180342
Patch inline:
diff --git a/common/autoconf/toolchain.m4 b/common/autoconf/toolchain.m4
--- a/common/autoconf/toolchain.m4
+++ b/common/autoconf/toolchain.m4
@@ -1000,6 +1000,8 @@
      fi
    fi

+  BASIC_FIXUP_EXECUTABLE(JTREGEXE)
+  BASIC_FIXUP_PATH(JT_HOME)
    AC_SUBST(JT_HOME)
    AC_SUBST(JTREGEXE)
  ])

/Magnus