The reason for this is that the compiler itself is built
using setjump/longjump style exceptions which is not
supported by LLVM (normal code built by the compiler uses
dwarf zero-cost style exceptions which are supported by LLVM).
Exceptions are raised inside the compiler when it builds
the runtime (due to processing files containing pragmas
for VAX) and need to be caught.  The exceptions are caught
if the new compiler was built by the system compiler, but
not if it was built using the llvm-gcc Ada compiler, which
is what happens during bootstrap.

This could be fixed by adding sj/lj exception support to
LLVM, or by modifying the build system so that the compiler
uses dwarf zcx internally.