target/riscv: Reuse tb->flags.FS

target/riscv: Reuse tb->flags.FS

When misa.F is 0 tb->flags.FS field is unused and can be used to save
the current state of smstateen0.FCSR check which is needed by the
floating point translation routines.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Message-Id: <20230518175058.2772506-3-mchitale@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>