pv: fix build error on sparc/sparc64

Relax linker option is enabled by default on sparc/sparc64
architectures, and it can't be used together with -r option, so disable
it.

Fixes:
http://autobuild.buildroot.net/results/fadc6fc5500d68b91aead58b1d74887db5244f52/
http://autobuild.buildroot.net/results/2f30486f2d72e0277fc30febd4012825a59e4f41/
http://autobuild.buildroot.net/results/82ccb8f0818d1909309eaef969548f963fc6cf04/


...

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>