x86/bugs: Rename CONFIG_GDS_FORCE_MITIGATION => CONFIG_MITIGATION_GDS_FORCE

So the CPU mitigations Kconfig entries - there's 10 meanwhile - are named
in a historically idiosyncratic and hence rather inconsistent fashion
and have become hard to relate with each other over the years:

   https://lore.kernel.org/lkml/20231011044252.42bplzjsam3qsasz@treble/



When they were introduced we never expected that we'd eventually have
about a dozen of them, and that more organization would be useful,
especially for Linux distributions that want to enable them in an
informed fashion, and want to make sure all mitigations are configured
as expected.

For example, the current CONFIG_SPECULATION_MITIGATIONS namespace is only
halfway populated, where some mitigations have entries in Kconfig, and
they could be modified, while others mitigations do not have Kconfig entries,
and can not be controlled at build time.

Fine-grained control over these Kconfig entries can help in a number of ways:

  1) Users can choose and pick only mitigations that are important for
     their workloads.

  2) Users and developers can choose to disable mitigations that mangle
     the assembly code generation, making it hard to read.

  3) Separate Kconfigs for just source code readability,
     so that we see *which* butt-ugly piece of crap code is for what
     reason...

In most cases, if a mitigation is disabled at compilation time, it
can still be enabled at runtime using kernel command line arguments.

This is the first patch of an initial series that renames various
mitigation related Kconfig options, unifying them under a single
CONFIG_MITIGATION_* namespace:

    CONFIG_GDS_FORCE_MITIGATION => CONFIG_MITIGATION_GDS_FORCE
    CONFIG_CPU_IBPB_ENTRY       => CONFIG_MITIGATION_IBPB_ENTRY
    CONFIG_CALL_DEPTH_TRACKING  => CONFIG_MITIGATION_CALL_DEPTH_TRACKING
    CONFIG_PAGE_TABLE_ISOLATION => CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
    CONFIG_RETPOLINE            => CONFIG_MITIGATION_RETPOLINE
    CONFIG_SLS                  => CONFIG_MITIGATION_SLS
    CONFIG_CPU_UNRET_ENTRY      => CONFIG_MITIGATION_UNRET_ENTRY
    CONFIG_CPU_IBRS_ENTRY       => CONFIG_MITIGATION_IBRS_ENTRY
    CONFIG_CPU_SRSO             => CONFIG_MITIGATION_SRSO
    CONFIG_RETHUNK              => CONFIG_MITIGATION_RETHUNK

Implement step 1/10 of the namespace unification of CPU mitigations related
Kconfig options and rename CONFIG_GDS_FORCE_MITIGATION to
CONFIG_MITIGATION_GDS_FORCE.

[ mingo: Rewrote changelog for clarity. ]

Suggested-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20231121160740.1249350-2-leitao@debian.org