hw/riscv: Add signature dump function for spike to run ACT tests

hw/riscv: Add signature dump function for spike to run ACT tests

Add signature and signature-granularity properties in spike to specify the target
signatrue file and the line size for signature data.

Recgonize the signature section between begin_signature and end_signature symbols
when loading elf of ACT tests. Then dump signature data in signature section just
before the ACT tests exit.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230405095720.75848-2-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>