os, syscall: move rlimit code to syscall

os, syscall: move rlimit code to syscall

In CL 393354 the os package was changed to raise the open file rlimit
at program start. That code is not inherently tied to the os package.
This CL moves it into the syscall package.

This is in preparation for future changes to restore the original
soft rlimit when exec'ing a new program.

For #46279

Change-Id: I981401b0345d017fd39fdd3dfbb58069be36c272
Reviewed-on: https://go-review.googlesource.com/c/go/+/476096
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>