[llvm] [Support] Revert "Reimplement getMainExecutable() using sysctl on NetBSD"

This apparently does not work reliably after all (non-reentrant?)
and causes test failures such as:

http://lab.llvm.org:8011/builders/netbsd-amd64/builds/19254/steps/run%20unit%20tests/logs/FAIL%3A%20libc%2B%2B%3A%3Asize.pass.cpp

llvm-svn: 355302