Add support for RTEMS 5.1 - Cmake updates and pc-rtems PSP readme updates

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Is your feature request related to a problem? Please describe.
The cFS Bundle currently supports RTEMS 4.11. Now that RTEMS 5.1 has been released, I would like to update the necessary components to support RTEMS 5.1 on the pc-rtems platform. This involves minor modifications to the cfe repository, the PSP repository, and the OSAL repository. These changes can be done in such a way that preserves the current RTEMS 4.11 support and adds RTEMS 5.1 support.

Describe the solution you'd like
I would like to add support for RTEMS 5.1 while maintaining compatibility with the existing RTEMS 4.11 support. For the PSP repository there are a few minor changes needed to allow support of RTEMS 5.1 and RTEMS 4.11:

• Minor changes in the PSP cmake/Modules/Platform/RTEMS.cmake file
• changes to the pc-rtems readme and the addition of an RTEMS 5 specific readme

Describe alternatives you've considered
Alternatives include:

• Not supporting newer versions of RTEMS, but several projects will depend on RTEMS 5 support.
• Dropping RTEMS 4.11 support and just making the changes needed for RTEMS 5, but there may be projects that depend on RTEMS 4.11. We can consider dropping RTEMS 4.11 support on a future release.

Additional context

Requester Info
Alan Cudmore / NASA GSFC Code 582

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Related to
cFE 914: nasa/cFE#914
OSAL 608: nasa/osal#608