C structs travel as C passes them: up to 16 bytes in INTEGER/SSE registers per eightbyte,
bigger ones byval and through sret, out-of-registers structs on the stack. addr(fn) makes a
C-callable thunk that rebuilds the arguments. With those, the self-hosted compiler can
compile boot/cimport.fch, which calls libclang with CXCursor and CXString by value and gives
it callbacks -- so it compiles itself.
boot.sh: stage 2 and stage 3 are identical (57904 lines of IR); 22 passed, 0 failed,
0 skipped. Only x86-64 System V so far; Windows and ARM64 conventions are still in C++.
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