cmake: gcc: target_arm: Force FP hard ABI for DCLS

When Dual-redundant Core Lock-step (DCLS) topology is used, the VFP
registers across the two redundant cores must be manually initialised
and synchronised, and this requires the `-mfloat-abi=hard` option to
be specified.

This commit forces the use of FP "hard" ABI on the VFP-equipped cores
that are configured in DCLS topology.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>