[SPARK-19999][BACKPORT-2.1][CORE] Workaround JDK-8165231 to identify PPC64...

[SPARK-19999][BACKPORT-2.1][CORE] Workaround JDK-8165231 to identify PPC64 architectures as supporting unaligned access

## What changes were proposed in this pull request?

This PR is backport of #17472 to Spark 2.1

java.nio.Bits.unaligned() does not return true for the ppc64le arch.
see [https://bugs.openjdk.java.net/browse/JDK-8165231](https://bugs.openjdk.java.net/browse/JDK-8165231)
Check architecture in Platform.java

## How was this patch tested?

unit test

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #17509 from kiszk/branch-2.1.

common/unsafe/src/main/java/org/apache/spark/unsafe/Platform.java

@@ -46,18 +46,22 @@ public final class Platform {
   private static final boolean unaligned;
   static {
     boolean _unaligned;
-    // use reflection to access unaligned field
-    try {
-      Class<?> bitsClass =
-        Class.forName("java.nio.Bits", false, ClassLoader.getSystemClassLoader());
-      Method unalignedMethod = bitsClass.getDeclaredMethod("unaligned");
-      unalignedMethod.setAccessible(true);
-      _unaligned = Boolean.TRUE.equals(unalignedMethod.invoke(null));
-    } catch (Throwable t) {
-      // We at least know x86 and x64 support unaligned access.
-      String arch = System.getProperty("os.arch", "");
-      //noinspection DynamicRegexReplaceableByCompiledPattern
-      _unaligned = arch.matches("^(i[3-6]86|x86(_64)?|x64|amd64|aarch64)$");
+    String arch = System.getProperty("os.arch", "");
+    if (arch.equals("ppc64le") || arch.equals("ppc64")) {
+      // Since java.nio.Bits.unaligned() doesn't return true on ppc (See JDK-8165231), but ppc64 and ppc64le support it
+      _unaligned = true;
+    } else {
+      try {
+        Class<?> bitsClass =
+          Class.forName("java.nio.Bits", false, ClassLoader.getSystemClassLoader());
+        Method unalignedMethod = bitsClass.getDeclaredMethod("unaligned");
+        unalignedMethod.setAccessible(true);
+        _unaligned = Boolean.TRUE.equals(unalignedMethod.invoke(null));
+      } catch (Throwable t) {
+        // We at least know x86 and x64 support unaligned access.
+        //noinspection DynamicRegexReplaceableByCompiledPattern
+        _unaligned = arch.matches("^(i[3-6]86|x86(_64)?|x64|amd64|aarch64)$");
+      }
     }
     unaligned = _unaligned;
   }