Fix PDB relocation on big-endian hosts

When running the LLD test suite on a big-endian host, the
COFF/pdb-framedata.yaml test case currently fails.

As it turns out, this is because code in DebugSHandler::finish
intended to relocate RvaStart entries of FDO records does not
work correctly when compiled for a big-endian host.

Fixed by always reading file data in little-endian mode.

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D149268