misa reports incorrect value in Sail

[rmn30]

Sail currently sets bits C, D, E, F, M, V and X in misa. This is wrong as D (double float), F (float) and V (vector) are not supported. Also B (bit vector) is supported and should be set (removed as I don't think this is standardized).

The -F and -W options to the simulator can be used to disable the floating point and vector extensions, but we should change the default in sail-riscv/c_emulator/riscv_platform_impl.c as we don't even compile the instruction definitions.

[rmn30]

This upstream PR looks like it could help with this: riscv/sail-riscv#506