syscall: mark C syscall functions noescape

Many C syscall functions take pointer arguments. The pointers
    don't escape in the C functions. Mark the C functions noescape so
    calling them doesn't need allocation.
    
    Reviewed-on: https://go-review.googlesource.com/c/158158

From-SVN: r267989

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-9a79c333e896ea49f6a708d459148074d29a2af6
+ee94431c133a90ca5c3c5ebbebcb019c60258dac
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/syscall/exec_linux.go

@@ -62,6 +62,7 @@ func runtime_AfterFork()
 func runtime_AfterForkInChild()
 
 // Implemented in clone_linux.c
+//go:noescape
 func rawClone(flags _C_ulong, child_stack *byte, ptid *Pid_t, ctid *Pid_t, regs unsafe.Pointer) _C_long
 
 // Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.

libgo/go/syscall/mksyscall.awk

@@ -98,6 +98,7 @@ BEGIN {
     printf("// Automatically generated wrapper for %s/%s\n", gofnname, cfnname)
     if (!(cfnname in cfns)) {
         cfns[cfnname] = 1
+        printf("//go:noescape\n")
         printf("//extern %s\n", cfnname)
         printf("func c_%s(%s) %s\n", cfnname, cfnparams, cfnresult)
     }