build: 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)