arch/arm: add option for FPv4 FPU
The FPv4-SP FPU is a single-precision FPU with 16 double registers [0]
[1]. It is only available for cortex-m4 cores, and is known to gcc as
fpv4-sp-d16 (note that there is no leading 'v') since gcc-4.5 [2].
[0] https://en.wikipedia.org/wiki/ARM_Cortex-M#Cortex-M4
[1] https://developer.arm.com/docs/ddi0439/latest/floating-point-unit
[2] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=
639cb7b789a54bf78d6ae5e2644450f5eb1837a6
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>