[instrprof] Fix issue in binary-ids-padding.test

https://reviews.llvm.org/D135929 caused a failure in
binary-ids-padding.test in big endian configurations:
https://lab.llvm.org/buildbot/#/builders/231/builds/6709

binary-ids-padding.test writes the profile in little-endian format.
This patch changes the raw profile reader to use getDataEndianness()
instead of llvm::support::endian::system_endianness() to fix the issue.