MCP750 needs to set context size on exception handling

[jphickey]

Describe the bug
MCP750 is not storing the context size, which results in the size being 0 and therefore no extended data is saved to the log file even though its in memory.

To Reproduce
Found in unit testing with new PSP coverage tests (not yet merged)

Expected behavior
The CFE_PSP_ExceptionHook function should set the context_size field to the amount of data it has actually stored in the buffer. This is then used later if a file dump is requested to write the exception data to a file.

System observed on:
Ubuntu 20.04 running PSP coverage tests.

Reporter Info
Joseph Hickey, Vantage Systems, Inc.