Fix #424, re-add rtems_fxp_attach() to cfe_psp_start.c.

[dzbaker]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
Fixes #424.

Testing performed
Workflows run.

Expected behavior changes
Allows cFS to build for RTEMS 5 with OMIT_DEPRECATED=true

System(s) tested on

• Ubuntu 20.04

Contributor Info - All information REQUIRED for consideration of pull request
Dylan Z. Baker/NASA GSFC