fix __cxa_exception alignment under libstdc++ with gcc

Summary:
Fixes: facebook#2359.

In the debugger, I observed calls to `__cxa_allocate_exception` with size `n` calling `malloc` with size `128 + n`. I compared 128 with `sizeof(__cxa_refcounted_exception)` in our replica, which was only 120. So there must have been some difference between the replica and the original. But there was no difference between the fields in `__cxa_refcounted_exception` or in `__cxa_exception`; so the only remaining place to look was in `_Unwind_Exception`. We do not have a replica of `_Unwind_Exception` since `unwind.h` is always available, but it might be from either libgcc or from libunwind, so the difference must have been between these two.

* The type `_Unwind_Exception` is marked in libgcc as being fully aligned: https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/libgcc/unwind-generic.h#L106.
* Likewise in llvm-unwind: https://github.com/llvm/llvm-project/blob/llvmorg-19.1.7/libunwind/include/unwind_itanium.h#L41.
* It is used without explicit alignment in libstdc++ authoritative `__cxa_exception`: https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/libstdc%2B%2B-v3/libsupc%2B%2B/unwind-cxx.h#L97.
* The type may not be marked as being fully aligned in upstream libunwind: https://github.com/libunwind/libunwind/blob/v1.8.1/include/unwind.h#L80.
* In our replica of `__cxa_exception`, we must mark the field as being fully aligned in case the replica uses a definition of the type which is not marked as being fully aligned.

Differential Revision: D68391229

folly/lang/Exception.cpp

@@ -96,6 +96,17 @@ typedef unsigned _Unwind_Ptr __attribute__((__mode__(__pointer__)));
 
 namespace __cxxabiv1 {
 
+//  libsupc++ itanium-abi __cxa_exception assumes that _Unwind_Exception is
+//  maximally-aligned, and indeed libgcc itanium-abi _Unwind_Exception is so;
+//  but libunwind itanium-abi _Unwind_Exception is not maximally-aligned
+#ifdef __ARM_EABI_UNWINDER__
+//  https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/gcc/ginclude/unwind-arm-common.h#L119
+static constexpr size_t __folly_unwind_exception_align = 8;
+#else
+//  https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/libgcc/unwind-generic.h#L106
+static constexpr size_t __folly_unwind_exception_align = alignof(max_align_t);
+#endif
+
 static constexpr uint64_t __gxx_primary_exception_class =
     0x474E5543432B2B00; // GNCUC++\0
 static constexpr uint64_t __gxx_dependent_exception_class =
@@ -118,7 +129,7 @@ struct __cxa_exception {
   _Unwind_Ptr catchTemp;
   void* adjustedPtr;
 #endif
-  _Unwind_Exception unwindHeader;
+  alignas(__folly_unwind_exception_align) _Unwind_Exception unwindHeader;
 };
 
 struct __cxa_refcounted_exception {