x86/lam: Disable ADDRESS_MASKING in most cases

In the Linux kernel, the following vulnerability has been resolved: x86/lam: Disable ADDRESS_MASKING in most cases Linear Address Masking (LAM) has a weakness related to transient execution as described in the SLAM paper[1]. Unless Linear Address Space Separation (LASS) is enabled this weakness may be exploitable. Until kernel adds support for LASS[2], only allow LAM for COMPILE_TEST, or when speculation mitigations have been disabled at compile time, otherwise keep LAM disabled. There are no processors in market that support LAM yet, so currently nobody is affected by this issue. [1] SLAM: https://download.vusec.net/papers/slam_sp24.pdf [2] LASS: https://lore.kernel.org/lkml/20230609183632.48706-1-alexander.shishkin@linux.intel.com/ [ dhansen: update SPECULATION_MITIGATIONS -> CPU_MITIGATIONS ]