Add OSAL support for RTEMS 6

[ghost]

Is your feature request related to a problem? Please describe.
RTEMS 6 is currently under development in the main branch of the RTEMS git repository. There are a few minor changes that have to be addressed in the OSAL code to support RTEMS 6.

Describe the solution you'd like
Modify the OSAL RTEMS implementation and OSAL pc-rtems BSP to support RTEMS 6 while still supporting RTEMS 4.11 and RTEMS 5.
This is done with approximately 6 minor changes. In the existing code there are conditional blocks selecting either RTEMS 5 and defaulting to RTEMS 4.11. For example:

ifdef _RTEMS_5_
   RTEMS 5 specific statement or define
else
   RTEMS 4.11 specific statement or define
endif

To support RTEMS 6, these blocks can be flipped as so:

ifdef OS_RTEMS_4_DEPRECATED
   RTEMS 4.11 specific statement or define
else
   RTEMS 5 and 6 (and hopefully future versions)
endif

By flipping these blocks, RTEMS 5 and 6 can be supported in the else block.

Describe alternatives you've considered
Alternatives include not supporting RTEMS 6, or just supporting RTEMS 6, neither of which are ideal. This solution will continue to support RTEMS 4.11, RTEMS 5, and add support for RTEMS 6.

Additional context
These changes depend on a number of other changes:
nasa/cFE#2096
nasa/PSP#338
#1248
I will not submit a pull request for this fix until the above changes are all in the main branch of the cFS bundle.

Requester Info
Alan Cudmore, NASA/GSFC Code 582.0