strings,bytes: use inlineable function trampolines instead of linkname

Cleans things up quite a bit.

There's still a few more, like runtime.cmpstring, which might also
be worth fixing.

Change-Id: Ide18dd621efc129cc686db223f47fa0b044b5580
Reviewed-on: https://go-review.googlesource.com/c/148578
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/strings/strings.go

@@ -146,6 +146,11 @@ func LastIndex(s, substr string) int {
 	return -1
 }
 
+// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
+func IndexByte(s string, c byte) int {
+	return bytealg.IndexByteString(s, c)
+}
+
 // IndexRune returns the index of the first instance of the Unicode code point
 // r, or -1 if rune is not present in s.
 // If r is utf8.RuneError, it returns the first instance of any