The generic benchmarking utilities added in PR 2125 did not compile on RISC-V due to two separate issues:

 * An incorrect format specifier for int32_t on RISC-V. This is resolved by using PRId32 instead of %ld or %d.
 * The fileno() is undeclared with the RISC-V toolchain, similar to how it is with the Embedded ARM toolchain. This PR resolves this by forgoing the fstat() check on file size (as this is what used fileno), and instead attempting to read the maximum number of bytes we can fit in the model buffer. We can then use feof() to verify that we read the entirety of the model, and error out if we did not. This solution also eliminates needing to worry about different file types, such as how the Xtensa simulator treats files from the host system as character devices.

BUG=2129