Undo commit cd359b25

Original text:
    Clarify use of |$end0| in stitched x86-64 AES-GCM code.

    There was some uncertainty about what the code is doing with |$end0|
    and whether it was necessary for |$len| to be a multiple of 16 or 96.
    Hopefully these added comments make it clear that the code is correct
    except for the caveat regarding low memory addresses.

    Change-Id: Iea546a59dc7aeb400f50ac5d2d7b9cb88ace9027
    Reviewed-on: https://boringssl-review.googlesource.com/7194


Reviewed-by: Adam Langley <agl@google.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3700)