prctl.2: Clarify the unsupported hardware case of EINVAL

prctl.2: Clarify the unsupported hardware case of EINVAL

prctls that are architecture-specific won't work on other
architectures, and arch-specific prctls that manipulate optional
hardware features likewise won't work if that hardware feature is
not present.

The established pattern seems to be to treat such prctls as if they
are unimplemented, when attempted on the wrong hardware.

Cover these cases with some generic weasel words in the closet
existing EINVAL clause.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>