libsodium: disable PIE on ARC

Even though ARC gcc understands "-pie" option and attempts to generate
PIE binaries as of today PIE is not really supported for user-space
applications.

So we disable PIE detection if building for ARC.
That fixes http://autobuild.buildroot.net/results/be4/be46b621ce5443788b0a1bc9fab614c4ca5d0859


and also prevents execution of non-supported PIE binary in runtime.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>