Revert "arm64: Do not invoke audit_syscall_* functions if !CONFIG_AUDIT_SYSCALL"

For some reason, the audit patches didn't make it out of -next this
merge window, so revert our temporary hack and let the audit guys deal
with fixing up -next.

This reverts commit 2a8f45b0.

Signed-off-by: Will Deacon <will.deacon@arm.com>