PR #1929: Fix shorten-64-to-32 warning in stacktrace_riscv-inl.inc

Imported from GitHub PR #1929

`ComputeStackFrameSize` returns `ptrdiff_t` which is `long` while `sizes[n]` is `int`.

This would become an error in chromium build as chromium enables warnings_as_error by default.

Merge ad1d680 into f040e96

Merging this change closes #1929

COPYBARA_INTEGRATE_REVIEW=#1929 from kxxt:riscv-int-conversion ad1d680
PiperOrigin-RevId: 802286490
Change-Id: I39526e7f9cc26302a18d329989a6b36ddd487f7c

Author: kxxt

absl/debugging/internal/stacktrace_riscv-inl.inc

@@ -162,7 +162,8 @@ static int UnwindImpl(void **result, uintptr_t *frames, int *sizes,
               absl::debugging_internal::StripPointerMetadata(frame_pointer);
         }
         if (sizes != nullptr) {
-          sizes[n] = ComputeStackFrameSize(frame_pointer, next_frame_pointer);
+          sizes[n] = static_cast<int>(
+              ComputeStackFrameSize(frame_pointer, next_frame_pointer));
         }
       }
       n++;