[X86] Change getHostCPUName to report Intel model 0x4e as "skylake" instead of "skylake-avx512". Add the proper 0x55 model for "skylake-avx512".
Intel's i5-6300U CPU is reporting to have a model id of 78 (4e).
The Host detection assumes that to be Skylake Xeon (with AVX512 support),
instead of a normal Skylake machine.
Patch by: Valentin Churavy
Reviewers: nalimilan, craig.topper
Subscribers: hfinkel, tkelman, craig.topper, nalimilan, llvm-commits
Differential Revision: https://reviews.llvm.org/D28221