disas/riscv: Use GString in format_inst

disas/riscv: Use GString in format_inst

Allocate and fill a GString instead of snprintf and
appending to a fixed sized buffer.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240412073346.458116-22-richard.henderson@linaro.org>