Revert part of r148839 and keep DefaultTargetTriple in the form adjusted

by -target and similar options. As discussed in PR 12026, the change
broke support for target-prefixed tools, i.e. calling x86_64--linux-ld
when compiling for x86_64--linux. Improve the test cases added
originally in r149083 to not require execution, just executable files.
Document the hack with appropiate FIXME comments.

llvm-svn: 151185