fix: Conditionally include `prctl.h` on macos

MacOS does not have the `sys/prctl.h` header. This patch conditionally
excludes `prctl.h` on MacOS and usage of its functionality in the same
way we currently do for windows.

Signed-off-by: Jack Frankland <jack.frankland@arm.com>
Change-Id: Ic56ec358552126b3a10827d9c52db388a8acc214