all: stop using indirect system calls on openbsd

[4a6f656c]

The indirect syscall call interface (syscall(2)) will be removed from OpenBSD, most likely before the 7.5 release. This effectively means that Go's syscall.Syscall* interfaces will no longer work on this platform and that direct system calls (using the appropriate libc system call stub) will need to be used instead.

[gopherbot]

Change https://go.dev/cl/538736 mentions this issue: syscall: call getfsstat via libc on openbsd

[gopherbot]

Change https://go.dev/cl/468095 mentions this issue: unix: use libc stubs for OpenBSD pledge+unveil

[gopherbot]

Change https://go.dev/cl/538995 mentions this issue: unix: use fcntl(2) libc stub on OpenBSD

[jrick]

I would perhaps reword this issue, as "direct system calls" could be misinterpreted to mean applications performing syscall instructions in their text, instead of using the particular libc stubs.

When we started requiring libc for system calls in dynamically linked executables, that was also referred to switching away from "direct system calls": #36435.

[gopherbot]

Change https://go.dev/cl/538976 mentions this issue: syscall: call getfsstat via libc on openbsd