[Support] Implement getMainExecutable on Solaris

Many `flang` tests currently `FAIL` on Solaris because the module files
aren't found.  I could trace this to `sys::fs::getMainExecutable` not being
implemented.

This patch does this and fixes all affected `flang` tests.

Tested on `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D109374