When throwing an Exception in the Fiber the backtrace generation
crashes.  This happens because backtrace does not func the stack bottom.
Using '.cfi_undefined x30' tells the debug info that the value in the lr
is unknown, which seems to be the nicest way to stop the unwinder.
Setting x30 to 0 is another option, however it still creates one invalid
frame in gdb, so the .cfi variant is used here instead.

Backport from upstream druntime 2.083.

Reviewed-on: https://github.com/dlang/druntime/pull/2308

From-SVN: r266470