[libunwind][cmake] Compile _Unwind* routines with -fexceptions (#121819)

When building libunwind with LTO, we found that routines, like
_Unwind_RaiseException were marked `nounwind`. This causes problems when
libunwind is then used with exception throwing code, since many of the
routines are marked `nounwind` and the compiler infers that something
like a try/catch block cannot throw resulting in a miscompile
(see #120657). Similarly, in #56825, it was pointed out that marking
_Unwind_Resume as `nounwind` causes bad exception table generation.

This patch adds the `-fexceptions` flag to the build of the C files that
define these routines, as proposed in #56825.

Fixes #56825 #120657

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Co-authored-by: Petr Hosek <phosek@google.com>

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