third_party/libunwind: Fix unwind ABI

`resumeParameters` in `_Unwind_FunctionContext` should be "unwind words"
(typically 64-bit) instead of 32-bit.

This fix, along with the pending libcxxabi port, allows the first
binaries with exception handling to properly run.

third_party/libunwind/README.cosmo

@@ -11,3 +11,6 @@ ORIGIN
   Author: Tobias Hieta <tobias@hieta.se>
   Date:   Tue, 28 Nov 2023 09:52:28 +0100
 
+LOCAL CHANGES
+  - Fixed `_Unwind_FunctionContext` struct to be ABI-compatible with code
+  generated by GCC.

third_party/libunwind/Unwind-sjlj.c

@@ -28,6 +28,8 @@
 
 #if defined(_LIBUNWIND_BUILD_SJLJ_APIS)
 
+typedef uintptr_t _Unwind_Word __attribute__((__mode__(__unwind_word__)));
+
 struct _Unwind_FunctionContext {
   // next function in stack of handlers
   struct _Unwind_FunctionContext *prev;
@@ -47,7 +49,7 @@ struct _Unwind_FunctionContext {
   uint32_t                        resumeLocation;
 
   // set by personality handler to be parameters passed to landing pad function
-  uint32_t                        resumeParameters[4];
+  _Unwind_Word                    resumeParameters[4];
 #endif
 
   // set by calling function before registering