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
Fangrui Song