[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

---------

Co-authored-by: Petr Hosek <[email protected]>

libunwind/src/CMakeLists.txt

@@ -20,7 +20,12 @@ set(LIBUNWIND_C_SOURCES
     )
 set_source_files_properties(${LIBUNWIND_C_SOURCES}
                             PROPERTIES
-                              COMPILE_FLAGS "-std=c99")
+                              # We need to set `-fexceptions` here so that key
+                              # unwinding functions, like
+                              # _UNWIND_RaiseException, are not marked as
+                              # `nounwind`, which breaks LTO builds of
+                              # libunwind.  See #56825 and #120657 for context.
+                              COMPILE_FLAGS "-std=c99 -fexceptions")
 
 set(LIBUNWIND_ASM_SOURCES
     UnwindRegistersRestore.S