ltp-testsuite: re-enable on AArch64

Since the bump of ltp-testsuite to a more recent version, the support
for AArch64 is now working correctly, so we can remove the exclusion
of this architecture from the ltp-testsuite package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>