Fix __builtin_bswap32/64 checks

* configure.ac (gcry_cv_have_builtin_bswap32)
(gcry_cv_have_builtin_bswap64): Change compile checks to link checks.

Patch changes compile checks to link checks for __builtin_bswap(32|64).
Compiling obviously works with missing functions, linking not so much.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>