[AIX] Remove diff -a option on llvm-cov.test

Summary:
llvm-cov.test generates .gcov files and compared with target
sample files. Since the files do not contain any binary data
(files are plain ASCII texts), remove -a from diff. And this
fix will the error on AIX since the default diff tool on AIX
does not support -a option.

Reviewers: hubert.reinterpretcast, daltenty, stevewan

Subscribers: llvm-commits

Tags: #LLVM

Differential Revision: https://reviews.llvm.org/D83711