utests: fix test case builtin_tgamma.
utests: fix test case builtin_tgamma.

Compare with tgamma instead of tgammaf for better accuracy.
Include negative inputs, and handle the resulting denormals.
Print maximum error found.

Signed-off-by: Rebecca Palmer <2decb937e7eca5e941ec22bc308fd452fe4ab2bb@zoho.com>
Reviewed-by: "Song, Ruiling" <286a174c1339c6eff91abc41c3a1012c95030c95@intel.com>
Reviewed-by: Zhigang Gong <e04a7b9b70b1e4c6318cf117dcd1a9056e14b97a@linux.intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet