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ARROW-7399: [C++][Gandiva] set Mcpu based on host cpu

- llvm doesn't auto detect cpu features based on host cpu. so,
  explicitly set the same in EngineBuilder.

Closes #6038 from pravindra/arrow-7399 and squashes the following commits:

f99694abe <Pindikura Ravindra> use static variable for getHostCpuName
7604eb0f8 <Pindikura Ravindra> ARROW-7399:  set Mcpu based on host cpu

Authored-by: Pindikura Ravindra <ravindra@dremio.com>
Signed-off-by: Pindikura Ravindra <ravindra@dremio.com>