Enable FPSIMD support on ARM64

Enable FPSIMD support on ARM64

Some of the functions implemented in musl can't work without the
ARM64 FPSIMD registers. This will fail unikraft build with musl
when FPSIMD is not enabled.
Enable FPSIMD by default when lib-musl is selected in menuconfig.

Signed-off-by: Razvan Virtan <virtanrazvan@gmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Robert Kuban <robert.kuban@opensynergy.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #14