RFR: JDK-8191205 Set native-debug-symbols default to "external"

[Magnus Ihse Bursie]

We should not have "zipped" as default for native-debug-symbols. In the 
future, this option should probably be dropped completely, but for now, 
start by not making it default.

Bug: https://bugs.openjdk.java.net/browse/JDK-8191205
Patch inline:
diff --git a/make/autoconf/jdk-options.m4 b/make/autoconf/jdk-options.m4
--- a/make/autoconf/jdk-options.m4
+++ b/make/autoconf/jdk-options.m4
@@ -296,13 +296,13 @@
        ],
        [
          if test "x$OPENJDK_TARGET_OS" = xaix; then
-          # AIX doesn't support 'zipped' so use 'internal' as default
+          # AIX doesn't support 'external' so use 'internal' as default
            with_native_debug_symbols="internal"
          else
            if test "x$STATIC_BUILD" = xtrue; then
              with_native_debug_symbols="none"
            else
-            with_native_debug_symbols="zipped"
+            with_native_debug_symbols="external"
            fi
          fi
        ])

/Magnus