Ignore *.o everywhere

We don't commit *.o files anywhere, not even as test data. So ignore them
everywhere.

This resolves *.o files not being ignored under 3rdparty/p256-m.

Also remove a redundant ignore of *.exe in a subdirectory.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.gitignore

@@ -28,6 +28,9 @@ massif-*
 .project
 /.settings
 
+# Unix-like build artifacts:
+*.o
+
 # MSVC build artifacts:
 *.exe
 *.pdb