skalibs: make ld use dummy file when configuring

skalibs: make ld use dummy file when configuring

For some architectures, like Xtensa or HPPA, ld from binutils requires
the output file to be a regular file, as mentioned in a bug report on
the mailing list [1].

So, use a dummy file as output file for ld, instead of /dev/null, when
trying to detect some libraries at configuration time.

Fixes http://autobuild.buildroot.net/results/288/288fc31cd10ffe3cd93371c7be37d79452a91768/

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=19526

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>