runtime/cgo: use //go:build lines in C and assembly files

runtime/cgo: use //go:build lines in C and assembly files

Replace deprecated // +build lines by their respective //go:build line
counterpart. Also remove build constraints implied by file name or type.

Change-Id: I8d18cd40071ca28d7654da8f0d22841f43729ca6
Reviewed-on: https://go-review.googlesource.com/c/go/+/460538
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>