syscall: wrap fstatat only once on linux/{arm64,riscv64}

syscall: wrap fstatat only once on linux/{arm64,riscv64}

Currently the linux/arm64 and linux/riscv64 ports wrap the fstatat
syscall twice, once in func fstatat and once in func Fstatat. Change the
latter to be a simple wrapper around the former to deduplicate some
code.

Change-Id: I82ec9374e2bcfe116eabf01f021eed1ee810ec15
Reviewed-on: https://go-review.googlesource.com/c/go/+/412934
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>