[BOLT] Adding a unittest that covers Arm SPE PBT aggregation (#160095)

When the SPE Previous Branch Target address (FEAT_SPE_PBT) feature is
available, an SPE sample by combining this PBT feature, has two entries.
Arm SPE records SRC/DEST addresses of the latest sampled branch
operation, and it stores into the first entry. PBT records the target
address of most recently taken branch in program order before the
sampled operation, it places into the second entry. They are formed a
chain of two consecutive branches.

Where:
- The previous branch operation (PBT) is always taken.
- In SPE entry, the current source branch (SRC) may be either
fall-through or taken, and the target address (DEST) of the recorded
branch operation is always what was architecturally executed.

However PBT doesn't provide as much information as SPE does. It lacks
those information such as the address of source branch, branch type, and
prediction bit. These information are always filled with zero in PBT
entry. Therefore Bolt cannot evaluate the prediction, and source branch
fields, it leaves them zero during the aggregation process.

Tests includes a fully expanded example.

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