[CUDA] Don't pass top-level -march down to device cc1 or ptxas.
Previously if you did e.g.
$ clang -march=haswell -x cuda foo.cu
we would pass "-march=haswell -march=sm_20" down to the ptxas tool.
This causes it to assert, and rightly so!
Subscribers: cfe-commits, echristo
Differential Revision: http://reviews.llvm.org/D21419