libspl: Fix incorrect use of platform defines on sparc64

libspl tries to detect sparc64 by checking whether sparc64
is defined. Unfortunately, this assumption is not correct as
sparc64 does not define sparc64 but it defines sparc
and arch64 instead. This leads to sparc64 being detected
as 32-Bit sparc and the build fails because both _ILP32 and
_LP64 are defined in this case.

To fix the problem, remove the checks for sparc64 and
just check arch64 if a sparc host was previously
detected with sparc.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Closes #5913