Add a NT_ARM_VFP ELF core note to hold VFP registers for each thread.

The core note matches the format and layout of NT_ARM_VFP on Linux.
Debuggers use the AT_HWCAP flags to determine how many VFP registers
are actually used and their format.

Reviewed by: mmel (earlier version w/o gcore)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D12293