Revert [llvm-objcopy][NFC] Refactor output target parsing

This reverts r364254 (git commit 545f001d)

This change causes some llvm-obcopy tests to fail with valgrind.

Following is the output for basic-keep.test
Command Output (stderr):
--

==107406== Conditional jump or move depends on uninitialised value(s)
==107406==    at 0x1A30DD: executeObjcopy(llvm::objcopy::CopyConfig const&) (llvm-objcopy.cpp:235)
==107406==    by 0x1A3935: main (llvm-objcopy.cpp:294)

llvm-svn: 364379