cgen: fix codegen for if comptime with -g

[felipensp]

Fix #22873

_{Huly®: V_0.6-21331}

[spytheman]

Excellent work.

[spytheman]

The test should be moved as a .vv/.out file pair under vlib/v/gen/c/testdata/ , that has this comment line at its top:
// vtest vflags: -g

It can print anything, for example println('done'), so that the .out file will not be empty.

Otherwise, -g is not passed by default, and the program compiles normally without it on master as well.