PR #1788: Fix warning for sign-conversion on riscv

Imported from GitHub PR #1788

```
error: operand of ? changes signedness: 'long' to 'uintptr_t' (aka 'unsigned long') [-Werror,-Wsign-conversion]
```

Merge 82deb4b into 95950b3

Merging this change closes #1788

COPYBARA_INTEGRATE_REVIEW=#1788 from luyahan:luyahan-patch-1 82deb4b
PiperOrigin-RevId: 702375910
Change-Id: Ia95d342ddd277dcbde19008e31aaef09f3c3a282

absl/debugging/internal/stacktrace_riscv-inl.inc

@@ -44,7 +44,8 @@ template <typename T>
 static inline ptrdiff_t ComputeStackFrameSize(const T *low, const T *high) {
   const char *low_char_ptr = reinterpret_cast<const char *>(low);
   const char *high_char_ptr = reinterpret_cast<const char *>(high);
-  return low < high ? high_char_ptr - low_char_ptr : kUnknownFrameSize;
+  return low < high ? static_cast<ptrdiff_t>(high_char_ptr - low_char_ptr)
+                    : kUnknownFrameSize;
 }
 
 // Given a pointer to a stack frame, locate and return the calling stackframe,