package/ljsyscall: fix build with aarch64_be

luajit supports aarch64_be since commit
28e5c2f2

However this raise a build failure with ljsyscall because aarch64_be
directory does not exist so use arm64 instead

/usr/bin/install: cannot stat '/home/buildroot/autobuild/instance-1/output-1/build/ljsyscall-0.12/syscall/linux/aarch64_be/*.lua': No such file or directory

Fixes:
 - http://autobuild.buildroot.org/results/78397c83e84dbfc09990f92be93e0b8a10d014c5



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>