Only include SetGPUArchs in the top-level CMakeLists.txt

[trxcllnt]

Fixes an issue seen when building both tests and benchmarks:

######################
# Configuring librmm #
######################
...
-- RMM: Building tests with GPU Architectures: 60-real;70-real;75-real;80
...
-- RMM: Building benchmarks with GPU Architectures: 60-real-real;70-real-real;75-real-real;80
-- Configuring done
CMake Error in benchmarks/CMakeLists.txt:
  Uknown CUDA architecture specifier "real-real".

[kkraus14]

@gpucibot merge

[kkraus14]

@gpucibot merge