For PR722:

Change the check for llvm-gcc from using LLVMGCCDIR to LLVMGCC. This checks
for the actual tool rather than the directory in which the tool resides. In
the case of this bug, it is possible that the directory exists but that the
tools in that directory do not. This fix should avoid the makefile from
erroneously proceeding without the actual tools being available.

llvm-svn: 27361