llvm-ar behaves weidly when creating thin archive. This only manifests
itself when using arduino sketches as these are built from the "bin"
directory.
Specifically, given a directory "m" and an object in "m/obj.o " an
invocation with CWD==m:
```
llvm-ar rcTs ../m.a obj.o
```
Will create a maformed archive. Binutils does not have any issue with this.
The following command, executed with CWD==m/.. works:
```
llvm-ar rcTs m.a m/obj.o
```
The trick used in this commit is to put the source files in a different
directory than the object files and compile from there.
72f934f13d