[SPARK-16922] [SPARK-17211] [SQL] make the address of values portable in LongToUnsafeRowMap

## What changes were proposed in this pull request?

In LongToUnsafeRowMap, we use offset of a value as pointer, stored in a array also in the page for chained values. The offset is not portable, because Platform.LONG_ARRAY_OFFSET will be different with different JVM Heap size, then the deserialized LongToUnsafeRowMap will be corrupt.

This PR will change to use portable address (without Platform.LONG_ARRAY_OFFSET).

## How was this patch tested?

Added a test case with random generated keys, to improve the coverage. But this test is not a regression test, that could require a Spark cluster that have at least 32G heap in driver or executor.

Author: Davies Liu <davies@databricks.com>

Closes #14927 from davies/longmap.