[AIX] Ignore case when comparing output from od

POSIX does not define the exact output from od tool.
While most implementations use lower case characters in hex output,
the z/OS USS implementation uses upper case characters.
To avoid LIT failures, the FileCheck option to ignore the case must
be used when checking hex bytes.

Reviewed By: abhina.sreeskantharajan

Differential Revision: https://reviews.llvm.org/D111427