Fix compilation error with devtoolset-8

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}

test/cctest/parsing/test-scanner-streams.cc

@@ -331,8 +331,8 @@ TEST(Utf8AdvanceUntilOverChunkBoundaries) {
   for (size_t i = 1; i < len; i++) {
     // Copy source string into buffer, splitting it at i.
     // Then add three chunks, 0..i-1, i..strlen-1, empty.
-    strncpy(buffer, unicode_utf8, i);
-    strncpy(buffer + i + 1, unicode_utf8 + i, len - i);
+    memcpy(buffer, unicode_utf8, i);
+    memcpy(buffer + i + 1, unicode_utf8 + i, len - i);
     buffer[i] = '\0';
     buffer[len + 1] = '\n';
     buffer[len + 2] = '\0';
@@ -360,8 +360,8 @@ TEST(Utf8ChunkBoundaries) {
   for (size_t i = 1; i < len; i++) {
     // Copy source string into buffer, splitting it at i.
     // Then add three chunks, 0..i-1, i..strlen-1, empty.
-    strncpy(buffer, unicode_utf8, i);
-    strncpy(buffer + i + 1, unicode_utf8 + i, len - i);
+    memcpy(buffer, unicode_utf8, i);
+    memcpy(buffer + i + 1, unicode_utf8 + i, len - i);
     buffer[i] = '\0';
     buffer[len + 1] = '\0';
     buffer[len + 2] = '\0';