cmd/compile, syscall: use go:norace comment for forkAndExecInChild

Use a go:norace comment rather than having the compiler know the special
name syscall.forkAndExecInChild.

Change-Id: I69bc6aa6fc40feb2148d23f269ff32453696fb28
Reviewed-on: https://go-review.googlesource.com/16097
Reviewed-by: Minux Ma <minux@golang.org>

src/syscall/exec_solaris.go

@@ -52,6 +52,7 @@ func write1(fd uintptr, buf uintptr, nbyte uintptr) (n uintptr, err Errno)
 // because we need to avoid lazy-loading the functions (might malloc,
 // split the stack, or acquire mutexes). We can't call RawSyscall
 // because it's not safe even for BSD-subsystem calls.
+//go:norace
 func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
 	// Declare all variables at top in case any
 	// declarations require heap allocation (e.g., err1).