[libspirv] Fix missing table declaration
These tables were split into head/tail tables upstream. The libspirv
versions of atan2/atan2pi were still using the old unified table, which
was never defined. A previous pulldown observed a build failure and
resolved it by adding a declaration of the old table; this table was
never defined so could result in a link failure at build time.
The correct fix is to have libspirv's atan2/atan2pi builtins call into
CLC's atan2/atan2pi, and at the same time remove the use of the old
tables from libspirv. We can now also remove the declaration of the
missing table.
