MFC 323581,323582,323583: Add ptrace operations for VFP registers.

323581:
Only mess with VFP state on the CPU for curthread for get/set_vfpcontext.

Future changes will use these functions to fetch and store VFP state for
threads other than curthread.

323582:
Add ptrace operations to fetch and store VFP registers.

323583:
Export get/set_vfpcontext from machdep.c.

Should have been part of the previous commit to add ptrace operations
for VFP registers.