arm64: Add support for PR_SPEC_DISABLE_NOEXEC prctl() option

The PR_SPEC_DISABLE_NOEXEC option to the PR_SPEC_STORE_BYPASS prctl()
allows the SSB mitigation to be enabled only until the next execve(),
at which point the state will revert back to PR_SPEC_ENABLE and the
mitigation will be disabled.

Add support for PR_SPEC_DISABLE_NOEXEC on arm64.

Reported-by: Anthony Steinhauser <asteinhauser@google.com>
Signed-off-by: Will Deacon <will@kernel.org>