Rollup merge of #60191 - gnzlbg:f16c, r=alexcrichton

Rollup merge of #60191 - gnzlbg:f16c, r=alexcrichton

Add f16c target_feature

This is requires for Intel 16-bit half-precision float intrinsics: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=fp16&expand=1769 - see companion stdsimd PR: https://github.com/rust-lang-nursery/stdsimd/pull/737.

LLVM, Wikipedia CPUID page, and the Intel Dev Manual all call this CPUID feature "F16C", but the Intel intrinsics guide calls this "FP16C" - this is probably a bug in the intrinsics guide which we are tracking here: https://github.com/rust-lang-nursery/stdsimd/issues/738

r? @alexcrichton