arm64: ptrace: Flush FPSIMD regs back to thread_struct before reading

When reading the FPSIMD state of current (which occurs when dumping
core), it is possible that userspace has modified the FPSIMD
registers since the time the task was last scheduled out.  Such
changes are not guaranteed to be reflected immedately in
thread_struct.

As a result, a coredump can contain stale values for these
registers.  Reading the registers of a stopped task via ptrace is
unaffected.

This patch explicitly flushes the CPU state back to thread_struct
before dumping when operating on current, thus ensuring that
coredump contents are up to date.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>