linux-headers: aarch64: Prevent selecting unsupported versions

Versions older than Linux v3.7 do not support the aarch64 architecture
so disable them, for reference see https://kernelnewbies.org/Linux_3.7



Without this patch these defconfigs fail to build

BR2_aarch64=y
BR2_KERNEL_HEADERS_3_2=y

BR2_aarch64=y
BR2_KERNEL_HEADERS_3_4=y

with error messages like this:

make[1]: Entering directory '/home/buildroot/br5_next/output/build/linux-headers-3.2.89'
Makefile:567: /home/buildroot/br5_next/output/build/linux-headers-3.2.89/arch/arm64/Makefile: No such file or directory

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>