Fix arm lldb-server on aarch64 device

* Use PTRACE_GETVFPREGS/PTRACE_SETVFPREGS to access the floating point
  registers instead of the old PTRACE_GETFPREGS/PTRACE_SETFPREGS. The
  new call is available since armv5.
* Work around a kernel issue in PTRACE_POKEUSER with reading out the full
  register set, modifying the neccessary value and then writing it back.

llvm-svn: 251111