Configurations/*.conf: ios targets face-lift.

Move ios targets to 15-ios.conf and modernize by deploying xcrun.
This excuses user from looking for paths and setting environment
variables. [Thanks to @0neday for hint.]

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6410)