wvstreams: disable with the ARC external toolchain

This toolchain lacks program_invocation_* in the uClibc configuration.

Fixes:

  http://autobuild.buildroot.org/results/653/6531009b2eae64b4e79f66a625cf2a3f9ffc2ac6/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>