Update the bn_div_rem_words comment

The comment says something about adding assembly for other ISAs, but it
seems most ISAs don't actually have double-wide division instructions.
(Despite this, the division-based BN_MONT_CTX_set still seems to beat
the Montgomery one on Arm. Less drastically than before
https://boringssl-review.googlesource.com/c/boringssl/+/60686, but
division still makes things faster.)

Also update the bug links post LLVM's GitHub migration. Finding the
corresponding GitHub issue is not always trivial.

Bug: 358687140
Change-Id: Iafb5118461a2c09c66840a44fbd257320a8d98b4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/70168
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/fipsmodule/bn/div.c

diff --git a/crypto/fipsmodule/bn/div.c b/crypto/fipsmodule/bn/div.c
index f524f89..ba6b69e 100644
--- a/crypto/fipsmodule/bn/div.c
+++ b/crypto/fipsmodule/bn/div.c
@@ -149,11 +149,11 @@
   //   * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65668
   //
   // Clang bugs:
-  //   * https://llvm.org/bugs/show_bug.cgi?id=6397
-  //   * https://llvm.org/bugs/show_bug.cgi?id=12418
+  //   * https://github.com/llvm/llvm-project/issues/6769
+  //   * https://github.com/llvm/llvm-project/issues/12790
   //
-  // These issues aren't specific to x86 and x86_64, so it might be worthwhile
-  // to add more assembly language implementations.
+  // These is specific to x86 and x86_64; Arm and RISC-V do not have double-wide
+  // division instructions.
 #if defined(BN_CAN_USE_INLINE_ASM) && defined(OPENSSL_X86)
   __asm__ volatile("divl %4"
                    : "=a"(*quotient_out), "=d"(*rem_out)