build: arm64 support; specifically use pre-generated openssl configs

This commit was specifically to support arm64 on the iPhone,
however instead of maintaining a specifically written version
of the opensslconf.h for this project, it should rely on
the openssl configure scripts to generate the correct configs.

This commit moves opensslconf.h -> opensslconf-generic.h to maintain
backwards compatibility and only affect the following platforms:

* iPhone arm64 (previously didn't work at all)
* iPhone armv7, armv7s
* iPhone i386 (the 32 bit simulator)
* iPhone x86_64 (the 64 bit simulator)

Note: The config files were generated with guidance from https://gist.github.com/foozmeat/5154962
If this style of config isn't acceptable for node.js, we can simply add `|| defined(__arm64__)` to this line which _seems_ to also be an acceptable fix.