Update RTEMS toolchain files for RTEMS 4.11, 5, and 6

[ghost]

Is your feature request related to a problem? Please describe.
This is part of the RTEMS updates described here: nasa/cFS#468
The RTEMS toolchain files require a few minor changes in order to:

• Add compatibility for RTEMS 6
• Allow both static and dynamically linked cFE core implementations
• Allow the Gaisler RCC toolchain to be used

Describe the solution you'd like
I would like do the following to existing RTEMS toolchain files:

• Add a define to indicate that the cFE core is dynamically linked
• Add a define to indicate the version of RTEMS being used. This is a new define, and when the proper OSAL and PSP changes have been made, the old defines will be removed. The new define has the form -DOS_RTEMS_X where the old define has the form -D_RTEMS_X_
• Add a RTEMS_BSP_SPECS variable to allow a version specific specs option to be specified. This allows the same CMake platform file for RTEMS to be used for RTEMS 4, 5, and 6.

I would also like to add a new toolchain file for i686-rtems6

Describe alternatives you've considered
The alternative is to create two additional Cmake platform files in the PSP for RTEMS 6 and Gaisler RCC 5. This is not ideal because there would be 3 separate RTEMS Cmake platform files to maintain and they would be almost the same.

Additional context
The toolchain file changes are the only changes needed in the cFE to support RTEMS 4.11, 5, and 6 as well as Gaisler RTEMS 5 (RCC 1.3.x). These changes also will enable a new generic-rtems OSAL BSP and PSP to be added later.

Requester Info
Alan Cudmore, NASA/GSFC Code 582.0

[astrogeco]

CCB:2022.04.27 Discussed