[X86] Optimize out a few extra strlen calls in getX86TargetCPU. NFCI

We had a conversion from const char * to StringRef and const char *
to std::string conversion. These both do their own
strlen call if the compiler doens't figure out how to share them.
By adding the temporary StringRef we can convert it to std::string
instead.

The other case is to use a StringSwitch<StringRef> instead of
StringSwitch<const char *> since the output values of the switch
are string literals. This allows the length to be computed at
compile time. Otherwise we have to convert from const char *
to std::string after the StringSwitch.