syscall: use correct type for TIOCSPGRP/TIOCGPGRP

These ioctls take a pid_t (generally a C integer aka int32) and not an int64 - we
currently get away with this on little endian 64 bit platforms, since the bytes
fall into the correct place, however this breaks on big endian 64 bit platforms
(like openbsd/mips64).

This is the same fix as CL 267605, however for libc based exec.

Updates #36435

Change-Id: I01ae4905cba5e1f8725fa6cb8c35403c511534b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/334881


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>