LKGS instruction is introduced with Intel FRED (flexible return and event
delivery) specification https://cdrdv2.intel.com/v1/dl/getContent/678938.
LKGS is independent of FRED, so we enable it as a standalone CPU feature.
LKGS behaves like the MOV to GS instruction except that it loads the base
address into the IA32_KERNEL_GS_BASE MSR instead of the GS segment’s
descriptor cache, which is exactly what Linux kernel does to load user level
GS base. Thus, with LKGS, there is no need to SWAPGS away from the kernel
Changes since V1:
* place fixup code into code section "__ex_table" instead of the obsoleted