target/riscv: rename ext_icbom to ext_zicbom

target/riscv: rename ext_icbom to ext_zicbom

Add a leading 'z' to improve grepping. When one wants to search for uses
of zicbom they're more likely to do 'grep -i zicbom' than 'grep -i
icbom'.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
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: <20231012164604.398496-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>