www/node: Update 7.10.0 -> 8.1.2

Bump to the latest major version of Node.js.

Remove files/patch-node.gyp, since this patch has been accepted
upstream. Remove
files/patch-deps_v8_src_base_atomicopsinternalsarm__gcc.h,
which patches a now non-existent file after a change in how V8
implements atomic operations. Conditionally add the
_LIBCPP_TRIVIAL_PAIR_COPY_CTOR define when using clang 3.x to
workaround a known problem in libc++.

Approved by: robak (mentor)
Differential Revision: https://reviews.freebsd.org/D11030