Enhance MIPS64 testing and simplify handling code for its peculiar relocations (...

Enhance MIPS64 testing and simplify handling code for its peculiar relocations (#300)

* Add handling for SHF_MASKOS section flags

* Add readelf testcases for MIPS64 specificities

* Simplify the decoding of MIPS64 relocations

Instead of using "fake" fields to parse the relocation structure and
then use complex shift/masks to recover the conveyed information (once
for big endian binaries, twice for little endian ones), use fields
actually described in the spec and use straightforward shifts to
synthetize the "fake" fields.