Support RTEMS 6, Gaisler RCC, and Dynamically loaded cFE build in RTEMS.cmake platform file

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Is your feature request related to a problem? Please describe.
A few minor modifications to the RTEMS.cmake platform file are needed to support RTEMS 6, Gaisler RCC (RTEMS 5), and Dynamically loaded cFE core builds.

Describe the solution you'd like
Modify cmake/Modules/Platform/RTEMS.cmake to add:
RTEMS_DYNAMIC_LOAD variable to select how the CMAKE executable rule is defined
Add a RTEMS_BSP_SPECS_FLAGS variable in the RTEMS_SYS_SPECS_FLAGS define to allow of toolchain file customizations.
This will allow existing RTEMS 4.11 and RTEMS 5 pc-rtems platforms to still build, while enabling RTEMS 6, RCC, and other RTEMS platforms.

Describe alternatives you've considered
The alternative here would be to use separate Cmake platform files, which creates more Cmake files to maintain and makes it more confusing to setup a new RTEMS target.

Additional context
Depends on the following cFE issue for minor toolchain file updates:
nasa/cFE#2096

Requester Info
Alan Cudmore, NASA/GSFC Code 582.0