[yaml2elf] - An attemp to fix s390x BB after r353607.

s390x is big-endian and seems r353607 had an issue with endianess,
Bot was unhappy:
http://lab.llvm.org:8011/builders/clang-s390x-linux-lnt/builds/11168/steps/ninja%20check%201/logs/stdio

This should fix it.

llvm-svn: 353613