[dsymutil] Make the triple detection more strict.

[dsymutil] Make the triple detection more strict.

MachOObjectFile offers a method for detecting the correct triple, use
it instead of the previous approximation. This doesn't matter right
now, but it will become important for mach-o universal (fat) binaries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243095 91177308-0d34-0410-b5e6-96231b3b80d8