target/riscv/cpu.c: use cpu_cfg_ext_auto_update() during realize()

target/riscv/cpu.c: use cpu_cfg_ext_auto_update() during realize()

Let's change the other instances in realize() where we're enabling an
extension based on a certain criteria (e.g. it's a dependency of another
extension).

We're leaving icsr and ifencei being enabled during RVG for later -
we'll want to error out in that case. Every other extension enablement
during realize is now done via cpu_cfg_ext_auto_update().

The end goal is that only cpu init() functions will handle extension
flags directly via "cpu->cfg.ext_N = true|false".

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230912132423.268494-17-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>