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Add a proper #define for the TSS selector instead of open coding 0x18 and
hoping future developers don't use that selector for something else.

Opportunistically rename the code and data selector macros to shorten the
names, align the naming with the kernel's scheme, and capture that they
are *kernel* segments.
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-17-seanjc@google.com

Signed-off-by: Sean Christopherson <seanjc@google.com>