vendor._gowin: fix comment syntax for add_preferences

[wipeseals]

Amaranth version

0.6.0.dev76

minimal program that demonstrates

run TangNano9kPlatform().build(AnyElaboratableClass(), do_program=True)

Warning: Invalid constraint: # (add_preferences placeholder)
1 warning, 0 errors

What you expected to happen, and what actually happened

Tracing the cause from the error logs, the problem is probably in the commented out text appended by default in add_preferences.

The IO constraints file (.cst) in the gowin platform handles commentouts with //, but add_preferences handles them with #.

I assume that it is probably mistaken for commentout in .ys.

// Automatically generated by Amaranth 0.6.0.dev76. Do not edit.
IO_LOC “clk27_0__io” 52;
IO_PORT “clk27_0__io” IO_TYPE=LVCMOS33;

(snip)

IO_LOC “button_1__io” 4; ...
IO_PORT “button_1__io” IO_TYPE=LVCMOS33;
IO_LOC “uart_0__tx__io” 17;
IO_PORT “uart_0__tx__io” PULL_MODE=UP;
IO_PORT “uart_0__tx__io” IO_TYPE=LVCMOS33;
# (add_preferences placeholder)

[whitequark]

LGTM, thanks!