Aarch64 SVE: Support changing vector lengths for ptrace
    
    When writing registers to the kernel, check if regcache VG has been changed. If
    so then update the thread's vector length. If required, resize the registers
    in the kernel dump before writing it back.
    
    When reading registers from the kernel, ensure regcache VG register is updated.
    The regcache registers should already be of the correct length.
    
    2018-09-13  Alan Hayward  <alan.hayward@arm.com>
    
    gdb/
    	* aarch64-linux-nat.c (fetch_sveregs_from_thread): Pass regcache.
    	(store_sveregs_to_thread): Likewise.
    	* nat/aarch64-sve-linux-ptrace.c (aarch64_sve_get_sveregs): Check
    	vector length.
    	(aarch64_sve_regs_copy_to_reg_buf): Likewise.
    	(aarch64_sve_regs_copy_from_reg_buf): Likewise.
    	* nat/aarch64-sve-linux-ptrace.h (aarch64_sve_get_sveregs): Add
    	reg_buf arg.