build: fix V8 build on FreeBSD

Use the -D_LIBCPP_TRIVIAL_PAIR_COPY_CTOR=1 flag because on FreeBSD
std::pairs copy constructor is non-trivial.

Refs: https://lists.freebsd.org/pipermail/freebsd-toolchain/2016-March/002094.html
Refs: https://github.com/gliaskos/freebsd-chromium/blob/master/www/chromium/Makefile#L202-L205

PR-URL: #12784
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

common.gypi

@@ -410,6 +410,9 @@
         'libraries': [ '-lelf' ],
       }],
       ['OS=="freebsd"', {
+        # Use this flag because on FreeBSD std::pairs copy constructor is non-trivial
+        # https://lists.freebsd.org/pipermail/freebsd-toolchain/2016-March/002094.html
+        'cflags': [ '-D_LIBCPP_TRIVIAL_PAIR_COPY_CTOR=1' ],
         'ldflags': [
           '-Wl,--export-dynamic',
         ],