gdb/csky support .reg2 for kernel 4.x and later

gdb/csky support .reg2 for kernel 4.x and later

When kernel's version >= 4.x, the size of .reg2 section will be 400.
Contents of .reg2 are {
    unsigned long vr[96];
    unsigned long fcr;
    unsigned long fesr;
    unsigned long fid;
    unsigned long reserved;
};

VR[96] means: (vr0~vr15) + (fr16~fr31), each Vector register is
128-bits, each Float register is 64 bits, the total size is
(4*96).

In addition, for fr0~fr15, each FRx is the lower 64 bits of the
corresponding VRx. So fr0~fr15 and vr0~vr15 regisetrs use the same
offset.