make 64bit fp reg d16-31 available on a32 backend

[robertmuth]

but only if those are available on a raspi 3

[robertmuth]

It still needs to be determined if the raspi3 actually has 32 64bit fp regs or only 16.

A raspberrypi 3 identifies its CPU as follows:

processor : 0
model name : ARMv7 Processor rev 4 (v7l)
BogoMIPS : 38.40
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 4