[compiler-rt] Use --standalone when running tests on the iOS simulator

We can use `simctl spawn --standalone` to enable running tests without
the need for an already-booted simulator instance.  This also side-steps
the problem of not having a good place to shutdown the instance after
we are finished with testing.

rdar://58118442

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D78409