[$] Spectre V1 defense in GCC

In many ways, Spectre variant 1 (the bounds-check bypass vulnerability) isthe ugliest of the Meltdown/Spectre set, despite being relatively difficultto exploit. Any given code base could be filled with V1 problems, but theyare difficult to find and defend against. Static analysis can help, butthe available tools are few, mostly proprietary, and prone to falsepositives. There is also a lack of efficient, architecture-independentways of addressing Spectre V1 in user-space code. As a result, only alimited effort (at most) to find and fix Spectre V1 vulnerabilities hasbeen made in most projects. An effort to add some defenses to GCC may helpto make this situation better, but it comes at a cost of its own.