syscall: mark C syscall functions noescape

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

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@267989 138bc75d-0d04-0410-961f-82ee72b054a4