runtime: factor out usesLibcall

Rather than inline lists of GOOS values, factor out the code that checks
if a runtime makes system calls via libcall.

Change-Id: Ib19d7e63a2b4b8314f1841c0ff26e1b3a16b4b22
Reviewed-on: https://go-review.googlesource.com/c/go/+/259239


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Go Bot <gobot@golang.org>