Finally, a patch was landed that indirectly fixed building with GCC (i.e. it was supposed to enable RISC-V support but happens to fix some code that was causing trouble with GCC).
I could not figure out a way to do a Clang build because any attempt will either be ignored by the Dart build tool or cause the build system to use the clang provided upstream by Dart. Since I'm not seeing any notable differences or problems, we can use GCC for now and switch to clang once I find out what is wrong.
Performance-wise: our package is only a tiny few seconds slower than the upstream version compiled by Clang. My testing on an Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz shows that dart-sass, one of the largest pure-Dart software I can find, compiles in 10.216s while the upstream version compiles in 9.955s. Similarly, the default AngularDart (a Dart web framework) example project compiles in 58.981s while the upstream version compiles in 58.654s. Therefore, I would say there is no severe performance lost here despite us using GCC.