add 'a' to chmod in llvm-lipo executability tests

Summary:
When specifying symbolic permissions with + or -, if none of
a/u/g/o are specified, bits set in the umask are not affected.
This caused the llvm-lipo executability tests to fail on some
systems, e.g. having an umask of 027 would cause chmod -x to not
clear the executable bit for others. This change instead
uses chmod a-x, which clears all the executable bits regardless
of umask.

Reviewers: smeenai, hans, anushabasana

Reviewed By: smeenai

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65342

llvm-svn: 367142