syscall: always use prlimit for getrlimit/setrlimit on Linux

syscall: always use prlimit for getrlimit/setrlimit on Linux

Linux added the prlimit system call in version 2.6.36.
As our minimum Linux kernel version is now 3.2,
simplify the various getrlimit/setlrimit implementations
to just always use prlimit.

For #67001

Change-Id: I2512c21c947d0bc83f8f9077c143163fd8d83be3
Reviewed-on: https://go-review.googlesource.com/c/go/+/609178
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>