Fix an inconsistent use of LLVMGCCDIR. In all other cases, this directory

refers to the "prefix" directory, i.e., one level above "bin".  LLVMGCCPATH
is used as the directory containing the llvm-gcc executable, so add a "/bin"
suffix to get from LLVMGCCDIR to LLVMGCCPATH.

llvm-svn: 66823