More x86_64 backend functions
vreg_eq()
x86_64 impl_undefined()
Check the return of out_* and v_*
x86_64 impl_op()
x86_64 impl_op_unary()
impl_*prologue/epilogue
x86_64 impl_load()
x86_64 reg <-> scratch index
x86_64 impl_store()
v_new_flag() and out_flush_volatile()
out_dbg_label()
x86_64 impl_return()
ICEing x86_64 impl_branch - cc1 now links
Label function prologues
TODO() out_new_*()
Support for numeric values
Support for global variables
Backend preserves types
Handle sym_arg:s and use type_pointed_to for new_sym()