Default to `-defaultlib=` (no druntime/Phobos) for *.c tests

[kinke]

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[kinke]

DMD codegen doesn't look too good (this works with LDC; some importC tests are disabled though). __builtin_va_end is apparently no builtin but a druntime import, and TypeInfos seem to be emitted into the supposedly C object files too.

[maxhaton]

ImportC.h is in druntime and will presumably need to be tested at some point when preprocessing is more final.

[kinke]

Filed https://issues.dlang.org/show_bug.cgi?id=23078.