cmake: gcc: Fix Cortex-R52 FPU type

The commit f10fa0de mapped the
Cortex-R52 processor type to use the "VFPv3" FPU type, but the
toolchain requires the "FPv5" FPU type and refuses to assemble
floating-point instructions when the "VFPv3" FPU type is specified.

This commit updates the build script to specify the FPU type of
`fpv5-sp-d16` when the processor is configured with a single-precision
FPU, and `neon-fp-armv8` when the processor is configured with a
double-precision + Advanced SIMD-capable FPU.

Note that the `fp-armv8` FPU type is an alias for double-precision FPv5
with 32 double-precision registers (refer to the GCC
`gcc/config/arm/arm-cpus.in` for more details); NEON is always
specified in case of a double-precision configuration because the
Cortex-R52 can only be configured as such.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>