[llvm-ar] Improve tool selection heuristic

If llvm-ar is installed at arm-pokymllib32-linux-gnueabi-llvm-ar, it may
think it is llvm-lib due to the "lib" substring.

Improve the heuristic to make all the following work as intended:

llvm-ar-9 (llvm-9 package on Debian)
llvm-ranlib.exe
Lib.exe (reported by D44808)
arm-pokymllib32-linux-gnueabi-llvm-ar (reported by D71030)

Reviewed By: raj.khem, rupprecht

Differential Revision: https://reviews.llvm.org/D71302