Make osl_getExecutableFile work even without a prior osl_setCommandArgs

After 2ad716f406e0fdb9b9294876c64ae92fecbf5e27 "Revert 'pyuno: set up fake
command line in getComponentContext(),'" e.g. PythonTest_sw_python would fail on
Windows, where WinSalGraphics::GetDevFontList (vcl/win/source/gdi/salgdi3.cxx)
calls osl_getExecutableFile and is itself called in a python process where
osl_setCommandArgs has not been set up.

This patch makes osl_getExecutableFile on all platforms if osl_setCommandArgs
has not (yet) been set fall back to the code that was
osl_bootstrap_getExecutableFile_Impl (which was called from
sal/rtl/bootstrap.cxx, which can now call osl_getExecutableFile).

Change-Id: I6c1bb59205041b3208c830a8b8406e28128b4566