Unhandled exception detail line on i386, arm32, riscv32 and xtensa

x86-64 and aarch64 (since 2026-10-20) print Unhandled exception: <ClassName>: <Message>. The other targets' blocks in compiler/exception_emit.inc still print only "Unhandled exception". Port the same guarded sequence. On the 32-bit targets the RTTI pointer sits at vmt-4 and msg at obj+4, so the offsets need checking per target, not copying. On ESP the line is what a serial console shows after a crash, which makes it worth the most there.