_mm_pause should not require the SSE2 target feature

[gnzlbg]

The spec is very likely incorrect (we should report this upstream): https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm_pause&expand=4108

(the SSE2 CPUID bit is not required)

Clang does not require it: https://github.com/llvm-mirror/clang/blob/be9413c753bdaff9cdb84b868f682abf6548ca60/include/clang/Basic/BuiltinsX86.def#L346

And the docs of pause state that it is backwards compatible with CPUs without the instructions, corresponding to a nop in those:

This instruction was introduced in the Pentium 4 processors, but is backward compatible with all IA-32 processors. In earlier IA-32 processors, the PAUSE instruction operates like a NOP instruction.

cc @Fanael