[traphandler] Add comment about section splitting

This adds a comment to explain why the additional "ret" instruction
added in https://crrev.com/c/3071202 is actually needed.

R=mseaborn@chromium.org
CC=ahaas@chromium.org

Bug: v8:11955
Change-Id: Ifd874c499fd3094f7cf5383e991d6b193b23ca63
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3081601Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76230}

src/trap-handler/handler-outside-simulator.cc

@@ -21,6 +21,11 @@ asm(
     "  movb (%rdi), %al                             \n"
     // Return 0 on success.
     "  xorl %eax, %eax                              \n"
+    // Place an additional "ret" here instead of falling through to the one
+    // below, because (some) toolchain(s) on Mac set ".subsections_via_symbols",
+    // which can cause the "ret" below to be placed elsewhere. An alternative
+    // prevention would be to add ".alt_entry" (see
+    // https://reviews.llvm.org/D79926), but just adding a "ret" is simpler.
     "  ret                                          \n"
     ".globl " SYMBOL(v8_probe_memory_continuation) "\n"
     SYMBOL(v8_probe_memory_continuation) ":         \n"