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

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>
Signed-off-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>