[dev.cc] all: merge dev.power64 (7667e41f3ced) into dev.cc

This is to reduce the delta between dev.cc and dev.garbage to just garbage collector changes.

These are the files that had merge conflicts and have been edited by hand:
        malloc.go
        mem_linux.go
        mgc.go
        os1_linux.go
        proc1.go
        panic1.go
        runtime1.go

LGTM=austin
R=austin
CC=golang-codereviews
https://golang.org/cl/174180043

src/runtime/string.go

@@ -39,22 +39,18 @@ func concatstrings(a []string) string {
 	return s
 }
 
-//go:nosplit
 func concatstring2(a [2]string) string {
 	return concatstrings(a[:])
 }
 
-//go:nosplit
 func concatstring3(a [3]string) string {
 	return concatstrings(a[:])
 }
 
-//go:nosplit
 func concatstring4(a [4]string) string {
 	return concatstrings(a[:])
 }
 
-//go:nosplit
 func concatstring5(a [5]string) string {
 	return concatstrings(a[:])
 }