x86/desc: Build boot_{,compat_}gdt[] in C

x86/desc: Build boot_{,compat_}gdt[] in C

... where we can at least get the compiler to fill in the surrounding space
without having to do it manually.  This also results in the symbols having
proper type/size information in the debug symbols.

Reorder 'raw' in the seg_desc_t union to allow for easier initialisation.

Leave a comment explaining the various restrictions we have on altering the
GDT layout.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Introduce SEL2GDT(). Correct GDT indices in public header comments.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>