RFR: 8278260: JavaFX shared libraries not stripped on Linux or macOS

[Kevin Rushforth]

Build change to strip the non-global symbols from native shared libraries on Linux and macOS by running `strip -x`, unless doing a `-PCONF=DebugNative` build.

Here is a before / after size comparison. All sizes in KBytes:

### Linux

| Native Library | Current | Stripped |
| --------------- | --------- | -------- |
| libavplugin-54.so | 52 | 48 |
| libavplugin-56.so | 52 | 48 |
| libavplugin-57.so | 52 | 48 |
| libavplugin-ffmpeg-56.so | 52 | 48 |
| libavplugin-ffmpeg-57.so | 52 | 48 |
| libavplugin-ffmpeg-58.so | 52 | 48 |
| libdecora_sse.so | 76 | 72 |
| libfxplugins.so | 56 | 52 |
| libglass.so | 16 | 12 |
| libglassgtk2.so | 932 | 324 |
| libglassgtk3.so | 932 | 324 |
| libgstreamer-lite.so | 2,280 | 2,100 |
| libjavafx_font.so | 20 | 16 |
| libjavafx_font_freetype.so | 28 | 28 |
| libjavafx_font_pango.so | 28 | 24 |
| libjavafx_iio.so | 148 | 140 |
| libjfxmedia.so | 2,048 | 516 |
| libjfxwebkit.so | 106,696 | 88,428 |
| libprism_common.so | 8 | 8 |
| libprism_es2.so | 64 | 64 |
| libprism_sw.so | 68 | 64 |

### macOS

| Native Library | Current | Stripped |
| --------------- | --------- | -------- |
| libdecora_sse.dylib | 88 | 88 |
| libfxplugins.dylib | 88 | 84 |
| libglass.dylib | 360 | 324 |
| libglib-lite.dylib | 1,192 | 1,148 |
| libgstreamer-lite.dylib | 1,708 | 1584 |
| libjavafx_font.dylib | 64 | 64 |
| libjavafx_iio.dylib | 308 | 300 |
| libjfxmedia.dylib | 212 | 200 |
| libjfxmedia_avf.dylib | 100 | 88 |
| libjfxwebkit.dylib | 85,896 | 71,636 |
| libprism_common.dylib | 20 | 20 |
| libprism_es2.dylib | 64 | 64 |
| libprism_sw.dylib | 88 | 88 |

-------------

Commit messages:
 - 8278260: JavaFX shared libraries not stripped on Linux or macOS

Changes: https://git.openjdk.java.net/jfx/pull/695/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=695&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278260
  Stats: 67 lines in 3 files changed: 64 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/695.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/695/head:pull/695

PR: https://git.openjdk.java.net/jfx/pull/695