arch: arm: aarch32: Add half-precision floating-point configs

This commit adds the half-precision (16-bit) floating-point
configurations to the ARM AArch32 architectures.

Enabling CONFIG_FP16 has the effect of specifying `-mfp16-format`
option (in case of GCC) which allows using the half-precision floating
point types such as `__fp16` and `_Float16`.

Note that this configuration can be used regardless of whether a
hardware FPU is available or supports half-precision operations.

When an FP16-capable FPU is not available, the compiler will
automatically provide the software emulations.

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