enable asm optimizations on darwin64-x86_64-cc target

It's likely to be safe to remove that flag entirely since the config
script should detect whether asm optimizations are appropriate for a
given platform, but this patch just enables it on a single platform
Merge pull request #1496 from reaperhulk/master

Enable OpenSSL ASM Optimizations
add __rvm_unload function, closes #1425
Merge branch 'master' of github.com:wayneeseguin/rvm