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This patch removes the tight loop that calls `plat_report_exception`
in unhandled exceptions in AArch64 state.
The new behaviour is to call the `plat_report_exception` only
once followed by call to `plat_panic_handler`.
This allows platforms to take platform-specific action when
there is an unhandled exception, instead of always spinning
in a tight loop.

Note: This is a subtle break in behaviour for platforms that
      expect `plat_report_exception` to be continuously executed
      when there is an unhandled exception.

Change-Id: Ie2453804b9b7caf9b010ee73e1a90eeb8384e4e8