Make symbolic constant generation more generic

Make symbolic constant generation more generic

The existing _GDTE_ATTR() macros are fine for GDT entries, but not much use
for anything else.

Replace the existing VAR_MACRO() with VAR_MACRO_C1(), which works in the same
way but passes a constant through, and use this to implement TOK_OR() which
works like _GDTE_ATTR() but takes a parameter rather than a hard coded
SEG_ATTR_.

Document the entities fully, and fix them to work correctly with empty
__VA_ARGS__.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>