[flang] Pass Argv0 to getIntriniscDir and getOpenMPHeadersDir (#73254)

The `llvm::sys::fs::getMainExecutable(nullptr, nullptr)` is not able to
obtain the correct executable path on AIX without Argv0 due to the lack
of a current process on AIX's `proc` filesystem. This causes a build
failure on AIX as intrinsic module directory is missing.

---------

Co-authored-by: Mark Danial <mak.danial@ibm.com>