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

[inglorion]

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.

[smeenai]

LGTM, thanks!

[anushabasana]

LG, thank you!

llvm/test/tools/llvm-lipo/create-executable.test

 # Executable testing is not supported on Windows, since all files are considered executable
 # UNSUPPORTED: system-windows
 # RUN: yaml2obj %p/Inputs/i386-slice.yaml > %t-i386.o
 # RUN: yaml2obj %p/Inputs/x86_64-slice.yaml > %t-x86_64.o
 
-# RUN: chmod -x %t-i386.o
-# RUN: chmod -x %t-x86_64.o
+# RUN: chmod a-x %t-i386.o
+# RUN: chmod a-x %t-x86_64.o
 # RUN: llvm-lipo %t-i386.o %t-x86_64.o -create -output %t-universal.o
 # RUN: test ! -x %t-universal.o
 
-# RUN: chmod +x %t-i386.o
+# RUN: chmod a+x %t-i386.o
 # RUN: llvm-lipo %t-i386.o %t-x86_64.o -create -output %t-universal.o
 # RUN: test -x %t-universal.o

llvm/test/tools/llvm-lipo/thin-executable-universal-binary.test

 # Executable testing is not supported on Windows, since all files are considered executable
 # UNSUPPORTED: system-windows
 # RUN: yaml2obj %s > %t-universal.o
 
-# RUN: chmod -x %t-universal.o
+# RUN: chmod a-x %t-universal.o
 # RUN: llvm-lipo %t-universal.o -thin i386 -output %t32.o
 # RUN: test ! -x %t32.o
 
-# RUN: chmod +x %t-universal.o
+# RUN: chmod a+x %t-universal.o
 # RUN: llvm-lipo %t-universal.o -thin i386 -output %t32-ex.o
 # RUN: test -x %t32-ex.o
 
 
 
 --- !fat-mach-o
 FatHeader:
   magic:           0xCAFEBABE