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

[jhb]

Summary

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.

Test Plan

This is part of a series to use VFP registers in gdb on armv7.

[jhb]

• Add NT_ARM_VFP support to gcore.