v: add full compiler support for _likely_(x)

vlib/builtin/cfns.c.v

@@ -1,10 +1,5 @@
 module builtin
 
-// See cheaders.v: _likely_ is actually a macro, to hint the C compiler
-// that the passed boolean expression is very likely to be true, so it
-// can generate assembly code, with less chance of branch misprediction.
-fn C._likely_(bool) bool
-
 // <string.h>
 fn C.memcpy(byteptr, byteptr, int) voidptr