Found by wbcheck

   WBCHECK ERROR: Missed write barrier detected!
     Parent object: 0x7b7b8487c450 (wb_protected: true)
       rb_obj_info_dump: 0x00007b7b8487c450 JSON/Generator/State/JSON::Ext::Generator::State JSON/Generator/State
     Reference counts - snapshot: 1, writebarrier: 0, current: 6, missed: 5
     Missing reference to: 0x7b7b82f35a10
       rb_obj_info_dump: 0x00007b7b82f35a10 T_STRING/String  len: 1, capa: 15 "1"
     Missing reference to: 0x7b7b82f35e90
       rb_obj_info_dump: 0x00007b7b82f35e90 T_STRING/String  len: 1, capa: 15 "2"
     Missing reference to: 0x7b7b83629e50
       rb_obj_info_dump: 0x00007b7b83629e50 T_STRING/String  len: 1, capa: 15 "3"
     Missing reference to: 0x7b7b83b62190
       rb_obj_info_dump: 0x00007b7b83b62190 T_STRING/String  len: 1, capa: 15 "4"
     Missing reference to: 0x7b7b83629490
       rb_obj_info_dump: 0x00007b7b83629490 T_STRING/String  len: 1, capa: 15 "5"

Fix missing write barrier on Generator State

Fix: #827

On very old compilers it might not exist, at that point
might as well skip SIMD entirely.

Don't assume `__builtin_cpu_supports` exists