target/riscv: implement MonitorDef HMP API

target/riscv: implement MonitorDef HMP API

The MonitorDef API is related to two HMP monitor commands: 'p' and 'x':

(qemu) help p
print|p /fmt expr -- print expression value (use $reg for CPU register access)
(qemu) help x
x /fmt addr -- virtual memory dump starting at 'addr'

For x86, one of the few targets that implements it, it is possible to
print the PC register value with $pc and use the PC value in the 'x'
command as well.

Those 2 commands are hooked into get_monitor_def(), called by
exp_unary() in hmp.c. The function tries to fetch a reg value in two
ways: by reading them directly via a target_monitor_defs array or using
a target_get_monitor_def() helper. In RISC-V we have *A LOT* of
registers and this number will keep getting bigger, so we're opting out
of an array declaration.

We're able to retrieve all regs but vregs because the API only fits an
uint64_t and vregs have 'vlen' size that are bigger than that.

With this patch we can do things such as:

- print CSRs and use their val in expressions:
(qemu) p $mstatus
0xa000000a0
(qemu) p $mstatus & 0xFF
0xa0

- dump the next 10 insn from virtual memory starting at x1 (ra):

(qemu) x/10i $ra
0xffffffff80958aea:  a9bff0ef          jal                     ra,-1382                # 0xffffffff80958584
0xffffffff80958aee:  10016073          csrrsi                  zero,sstatus,2
0xffffffff80958af2:  60a2              ld                      ra,8(sp)
0xffffffff80958af4:  6402              ld                      s0,0(sp)
0xffffffff80958af6:  0141              addi                    sp,sp,16
0xffffffff80958af8:  8082              ret
0xffffffff80958afa:  10016073          csrrsi                  zero,sstatus,2
0xffffffff80958afe:  8082              ret
0xffffffff80958b00:  1141              addi                    sp,sp,-16
0xffffffff80958b02:  e422              sd                      s0,8(sp)
(qemu)

Suggested-by: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250703130815.1592493-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>