[libunwind] Fix Unwind-EHABI.cpp:getByte on big-endian targets

Summary:
The function getByte is dependent on endianness and the current
behavior is incorrect on big-endian targets.

This patch fixes the issue.

Reviewers: phosek, ostannard, dmgreen, christof, chill

Reviewed By: ostannard, chill

Subscribers: chill, christof, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D64402

llvm-svn: 365505