runtime: fix sigfwd to not allocate memory

    
    The use of &[1]uintptr{fn} was causing sigfwd to allocate memory, even
    though it is being compiled for the runtime package.  That is a bad
    idea for this function, which is invoked by a signal handler.  Rewrite
    it to use only constructs that do not allocate memory when compiled
    for the runtime package.
    
    The test for this is misc/cgo/testcarchive in the main repo, which we
    don't yet test.
    
    Reviewed-on: https://go-review.googlesource.com/37454

From-SVN: r245777

gcc/go/gofrontend/MERGE

-0bcc1bc98dca48af40d9f54f4eacbbafaa30beb1
+e1502234b5011a1ab859519f1f217dbf4369ec9b
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/signal_gccgo.go

@@ -127,9 +127,10 @@ func raiseproc(sig uint32) {
 //go:nosplit
 //go:nowritebarrierrec
 func sigfwd(fn uintptr, sig uint32, info *_siginfo_t, ctx unsafe.Pointer) {
-	f1 := &[1]uintptr{fn}
-	f2 := *(*func(uint32, *_siginfo_t, unsafe.Pointer))(unsafe.Pointer(&f1))
-	f2(sig, info, ctx)
+	f1 := [1]uintptr{fn}
+	f2 := &f1
+	f3 := *(*func(uint32, *_siginfo_t, unsafe.Pointer))(unsafe.Pointer(&f2))
+	f3(sig, info, ctx)
 }
 
 //go:nosplit