x86/gsseg: Move load_gs_index() to its own new header file

GS is a special segment on x86_64, move load_gs_index() to its own new
header file to simplify header inclusion.

No change in functionality.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230112072032.35626-5-xin3.li@intel.com