[CodeGen] Disable -mfloat-abi hard option for LLVM < 6.0 (#4071)
[CodeGen] Disable -mfloat-abi hard option for LLVM < 6.0 (#4071)

The -mfloat-abi hard option does not work for LLVM < 6.0 as it is ignored.
This adds a fatal error when using unsupported LLVM versions so that the failure
is not silent.
sxjscience/tvm,sxjscience/tvm,sxjscience/tvm,tqchen/tvm,sxjscience/tvm,sxjscience/tvm,sxjscience/tvm,Laurawly/tvm-1,tqchen/tvm,dmlc/tvm,Laurawly/tvm-1,tqchen/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,tqchen/tvm,dmlc/tvm,tqchen/tvm,sxjscience/tvm,Laurawly/tvm-1,sxjscience/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,tqchen/tvm,dmlc/tvm,Laurawly/tvm-1,tqchen/tvm,tqchen/tvm,tqchen/tvm