We are compiling V8 using devtoolset-8 and it is generating a new
compilation error related to String Truncation:

error: ‘char* strncpy(char*, const char*, size_t)’ output truncated copying between 1 and 15 bytes from a string of length 15 [-Werror=stringop-truncation]
          strncpy(buffer, unicode_utf8, i);

Which basically means the null terminating character was not added to
the end of the buffer:
https://developers.redhat.com/blog/2018/05/24/detecting-string-truncation-with-gcc-8/

This CL will changes 2 uses of "strncpy" to "memcpy" as strings
are being copied partially and `\n` being added at a later stage.

Change-Id: I3656afb00463d70ddb8700a487a1978b793e1d09
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2155038Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#67277}