[X86] Make getX86TargetCPU return std::string instead of const char *. Remove...

[X86] Make getX86TargetCPU return std::string instead of const char *. Remove call to MakeArgString. NFCI

I believe this function used to be called directly from X86
specific code and was used to immediately create -target-cpu
command line. A later refactoring changed it to to be called from
a generic getCPU function that returns std::string. So on some
paths we created a string using MakeArgString converted that to
std::string then called MakeArgString again from that.

Instead just return std::string directly like the other targets.