[SPARK-17127] Make unaligned access in unsafe available for AArch64

## # What changes were proposed in this pull request?

From the spark of version 2.0.0 , when MemoryMode.OFF_HEAP is set , whether the architecture supports unaligned access or not is checked. If the check doesn't pass, exception is raised.

We know that AArch64 also supports unaligned access , but now only i386, x86, amd64, and X86_64 are included.

I think we should include aarch64 when performing the check.

## How was this patch tested?

Unit test suite

Author: Richael <Richael.Zhuang@arm.com>

Closes #14700 from yimuxi/zym_change_unsafe.

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

@@ -55,7 +55,7 @@ public final class Platform {
       // 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)$");
+      _unaligned = arch.matches("^(i[3-6]86|x86(_64)?|x64|amd64|aarch64)$");
     }
     unaligned = _unaligned;
   }