In looking at the linker, it appears that if the _llvm_prf_cnts section is explicitly placed by the linker command file, then the “start_llvm_prf_cnts" and "stop__llvm_prf_cnts" symbols are not created. 
The workaround is that the client either shouldn’t explicitly place the section, or if it does, the symbols should also be explicitly defined based on the section location.  If users don't explicitly place the section, the linker will place it based on the location of the .bss section.