By this time you would have resolved the issue, adding comments as it may be helpful for others.
_sp is defined in https://github.com/eclipse-threadx/threadx/blob/master/ports/cortex_r5/gnu/example_build/sample_threadx.ld
lscript.ld of vitis need to be updated from sample_threadx.ld to resolve the issue . Other way is create new varaible _sp in lscript.ld