target/riscv: use misa_mxl_max to populate isa string rather than TARGET_LONG_BITS

target/riscv: use misa_mxl_max to populate isa string rather than TARGET_LONG_BITS

A cpu may not have the same xlen as the compile time target, and
misa_mxl_max is the source of truth for what the hart supports.

The conversion from misa_mxl_max to xlen already has one user, so
introduce a helper and use that to populate the isa string.

Link: https://lore.kernel.org/qemu-riscv/20240108-efa3f83dcd3997dc0af458d7@orel/
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20240124-swear-monthly-56c281f809a6@spud>
[ Changes by AF:
 - Convert to use RISCVCPUClass *mcc
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>