[FIRRTL] Emit fopen calls to get fd's not mcd's.

[dtzSiFive]

Multichannel descriptors are limited to 30, where file descriptors usually have a much higher limit.

Specify opening files "w" which will truncate and is not binary; this may be a change depending on default behavior of mcd's.

[uenoku]

Thanks!

[dtzSiFive]

I think verilator uses "w" for this, although maybe "wb" for vpi_mcd_open, if I'm following this right:

fopen paths:
https://github.com/verilator/verilator/blob/1432bb8889ca444bb9475f4bc897dd7cb1d0ef58/include/verilated.cpp#L1603
which calls:
https://github.com/verilator/verilator/blob/1432bb8889ca444bb9475f4bc897dd7cb1d0ef58/include/verilated_imp.h#L280

vpi_mcd_open implementation:
https://github.com/verilator/verilator/blob/1432bb8889ca444bb9475f4bc897dd7cb1d0ef58/include/verilated_vpi.cpp#L3570-L3574