Xilinx FIR filter behavior
I heard back from the Xilinx people.

There's a C model for validating bit accuracy without timing.  See chapter 5 of the fir compiler manual in this link

https://www.xilinx.com/support/documentation/ip_documentation/fir_compiler/v7_2/pg149-fir-compiler.pdf

Their gcc instructions for compiling and running are accurate for rhel6. (in fact, their commands can be copied and pasted into bash.  Just make sure to add the working directory to the LD_LIBRARY_PATH)

Learning this library will be a little involved though.