[macho2yaml][test] Specify the object file's endianness. NFC.

This change specifies the endianness of the generated object file to fix
the testing failure.

Testing failure: http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/52449

Author: higuoxing

llvm/test/tools/obj2yaml/MachO/debug-aranges.yaml

@@ -100,6 +100,7 @@ DWARF:
 # RUN: yaml2obj --docnum=2 %s | obj2yaml | FileCheck %s -DSEGNAME=FOO --check-prefix=ARANGES
 
 --- !mach-o
+IsLittleEndian: true
 FileHeader:
   magic:      0xFEEDFACF
   cputype:    0x01000007