Bug 1110570: AVX instructions should be emitted only if XSAVE is enabled by the OS; r=sunfish

If the XSAVE functionality is disabled by the OS (which can happen in
virtualized environments, for instance), AVX instructions shouldn't be emitted
by the JITs. See also the assembly rountines for both x86 and x64 at
https://software.intel.com/en-us/blogs/2011/04/14/is-avx-enabled/

--HG--
extra : rebase_source : 9008ada71bfc33283578197e0257b30533c650b2
extra : amend_source : 5a796ac9d53b9104a19a7c2354c6201e3c77028d