..."target does not support '.file' without a number", which was introduced into
LLVM with <http://llvm.org/viewvc/llvm-project?view=revision&revision=349976>
"[MC] Enable .file support on COFF and diagnose it on unsupported targets",
stating:  "The 'single parameter' .file directive appears to be an ELF-only
featurea [sic] that is intended to insert the main source filename into the
string table table [sic]."

And <https://sourceware.org/binutils/docs-2.31/as/File.html> states about the
default single (file name) argument version:  "This statement may go away in
future: it is only recognized to be compatible with old as programs."

As external/python3 builds just fine on macOS with that .file directive removed,
lets just do that for now.

Change-Id: Ib28c29d0cacd151437447ccb2f6cfb8925e3e85a
Reviewed-on: https://gerrit.libreoffice.org/65704
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>