[llvm-debuginfo-analyzer] (08/09) - ELF Reader

The unitest and test cases are platform dependent (x86_64)
causing failures in:

  https://lab.llvm.org/buildbot/#/builders/245/builds/146
  https://lab.llvm.org/buildbot/#/builders/188/builds/21397
  No available targets are compatible with triple "x86_64-unknown-unknown".

Added:
- ';REQUIRES: x86-registered-target' to the LIT tests.
- Code to check if the target 'Triple::x86_64' is supported to
  the unittest case.