RISC-V: Add ptrace support for vectors

This patch add back the ptrace support with the following fix:
 - Define NT_RISCV_CSR and re-number NT_RISCV_VECTOR to prevent
   conflicting with gdb's NT_RISCV_CSR.
 - Use struct __riscv_v_regset_state to handle ptrace requests

Since gdb does not directly include the note description header in
Linux and has already defined NT_RISCV_CSR as 0x900, we decide to
sync with gdb and renumber NT_RISCV_VECTOR to solve and prevent future
conflicts.

Fixes: 0c59922c76 ("riscv: Add ptrace vector support")
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20230825050248.32681-1-andy.chiu@sifive.com
[Palmer: Drop the unused "size" variable in riscv_vr_set().]
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

include/uapi/linux/elf.h

@@ -443,6 +443,8 @@ typedef struct elf64_shdr {
 #define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers */
 #define NT_MIPS_FP_MODE	0x801		/* MIPS floating-point mode */
 #define NT_MIPS_MSA	0x802		/* MIPS SIMD registers */
+#define NT_RISCV_CSR	0x900		/* RISC-V Control and Status Registers */
+#define NT_RISCV_VECTOR	0x901		/* RISC-V vector registers */
 #define NT_LOONGARCH_CPUCFG	0xa00	/* LoongArch CPU config registers */
 #define NT_LOONGARCH_CSR	0xa01	/* LoongArch control and status registers */
 #define NT_LOONGARCH_LSX	0xa02	/* LoongArch Loongson SIMD Extension registers */