external/python3: Work around macOS Clang trunk error

..."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>

external/python3/UnpackedTarball_python3.mk

@@ -26,6 +26,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,python3,\
 	external/python3/python-3.3.5-pyexpat-symbols.patch.1 \
 	external/python3/ubsan.patch.0 \
 	external/python3/python-3.5.tweak.strip.soabi.patch \
+	external/python3/darwin.patch.0 \
 ))
 
 ifneq ($(filter DRAGONFLY FREEBSD LINUX NETBSD OPENBSD SOLARIS,$(OS)),)

external/python3/darwin.patch.0

+--- Modules/_ctypes/libffi_osx/x86/darwin64.S
++++ Modules/_ctypes/libffi_osx/x86/darwin64.S
+@@ -29,7 +29,6 @@
+ #include <fficonfig.h>
+ #include <ffi.h>
+ 
+- .file "darwin64.S"
+ .text
+ 
+ /* ffi_call_unix64 (void *args, unsigned long bytes, unsigned flags,