tcg/riscv: Probe for Zba, Zbb, Zicond extensions

tcg/riscv: Probe for Zba, Zbb, Zicond extensions

Define a useful subset of the extensions.  Probe for them
via compiler pre-processor feature macros and SIGILL.

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>