src/runtime: improve float/complex hashing

This allows positive and negative zero to hash to the same value,
as required by Go.

This is not perfect, but the best I could do without
revamping all the hash funtions to take a seed.

Fixes #2356

testdata/map.go

@@ -1,6 +1,8 @@
 package main
 
-import "sort"
+import (
+	"sort"
+)
 
 var testmap1 = map[string]int{"data": 3}
 var testmap2 = map[string]int{
@@ -109,6 +111,48 @@ func main() {
 	squares = make(map[int]int, 20)
 	testBigMap(squares, 40)
 	println("tested growing of a map")
+
+	floatcmplx()
+}
+
+func floatcmplx() {
+
+	var zero float64
+	var negz float64 = -zero
+
+	// test that zero and negative zero hash to the same thing
+	m := make(map[float64]int)
+	m[zero]++
+	m[negz]++
+	println(m[negz])
+
+	cmap := make(map[complex128]int)
+
+	var c complex128
+	c = complex(zero, zero)
+	cmap[c]++
+
+	c = complex(negz, negz)
+	cmap[c]++
+
+	c = complex(0, 0)
+	println(cmap[c])
+
+	c = complex(1, negz)
+	cmap[c]++
+
+	c = complex(1, zero)
+	cmap[c]++
+
+	println(cmap[c])
+
+	c = complex(negz, 2)
+	cmap[c]++
+
+	c = complex(zero, 2)
+	cmap[c]++
+
+	println(cmap[c])
 }
 
 func readMap(m map[string]int, key string) {

testdata/map.txt

@@ -72,3 +72,7 @@ structMap[{"tau", 3.14}]: 0
 structMap[{"tau", 6.28}]: 0
 tested preallocated map
 tested growing of a map
+2
+2
+2
+2