[compiler-rt] Provide better llvm-lit failure when llvm-config fails.

Summary:
The current error message can cause confusion if llvm-config can't be
executed for reasons other than "not found". In my example, cross
compiling generated an llvm-config binary for aarch64 which couldn't be
executed natively. Instead of telling me that the error was with the
file architecture, it reports the file as not being present.

Reviewers: pcc

Subscribers: dberris, javed.absar, kristof.beyls, jdoerfert, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58647

llvm-svn: 354907