runtime: replace index and contains with bytealg calls

The runtime has its own implementation of string indexing. To reduce
code duplication and cognitive load, replace this with calls to the
internal/bytealg package. We can't do this on Plan 9 because it needs
string indexing in a note handler (which isn't allowed to use the
optimized bytealg version because it uses SSE), so we can't just
eliminate the index function, but this CL does down-scope it so make
it clear it's only for note handlers on Plan 9.

Change-Id: Ie1a142678262048515c481e8c26313b80c5875df
Reviewed-on: https://go-review.googlesource.com/c/go/+/244537
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/os_plan9.go

@@ -82,10 +82,10 @@ func sigpanic() {
 	note := gostringnocopy((*byte)(unsafe.Pointer(g.m.notesig)))
 	switch g.sig {
 	case _SIGRFAULT, _SIGWFAULT:
-		i := index(note, "addr=")
+		i := indexNoFloat(note, "addr=")
 		if i >= 0 {
 			i += 5
-		} else if i = index(note, "va="); i >= 0 {
+		} else if i = indexNoFloat(note, "va="); i >= 0 {
 			i += 3
 		} else {
 			panicmem()
@@ -111,6 +111,20 @@ func sigpanic() {
 	}
 }
 
+// indexNoFloat is bytealg.IndexString but safe to use in a note
+// handler.
+func indexNoFloat(s, t string) int {
+	if len(t) == 0 {
+		return 0
+	}
+	for i := 0; i < len(s); i++ {
+		if s[i] == t[0] && hasPrefix(s[i:], t) {
+			return i
+		}
+	}
+	return -1
+}
+
 func atolwhex(p string) int64 {
 	for hasPrefix(p, " ") || hasPrefix(p, "\t") {
 		p = p[1:]