implement unsigned -> float casts

amd64 lacks an instruction for this so it has to be implemented with
signed -> float casts:
 - Word casting is done by zero-extending the word to a long and then doing
   a regular signed cast.
 - Long casting is done by dividing by two with correct rounding if the
   highest bit is set and casting that to float, then adding
   1 to mantissa with integer addition

arm64/emit.c

@@ -91,7 +91,9 @@ static struct {
 	{ Ostosi,  Ka, "fcvtzs %=, %S0" },
 	{ Odtosi,  Ka, "fcvtzs %=, %D0" },
 	{ Oswtof,  Ka, "scvtf %=, %W0" },
+	{ Ouwtof,  Ka, "ucvtf %=, %W0" },
 	{ Osltof,  Ka, "scvtf %=, %L0" },
+	{ Oultof,  Ka, "ucvtf %=, %L0" },
 	{ Ocall,   Kw, "blr %L0" },
 
 	{ Oacmp,   Ki, "cmp %0, %1" },