nv50/ir: support different unordered_set implementations

nv50/ir: support different unordered_set implementations

If build with C++11 standard, use std::unordered_set.

Otherwise if build on old Android version with stlport,
use std::tr1::unordered_set with a wrapper class.

Otherwise use std::tr1::unordered_set.

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>