[NFCI] [Darwin] Add APPLE_TARGET_IS_HOST lit option to compiler-rt (#174522)

Darwin supports running tests on targets other than the host machine
e.g. simulators. In such configurations, tests are run via a wrapper
script (e.g. iossim_run.py).

This refactors the lit test config to use a dedicated option to
distinguish test suites that run on the host from those that run in any
other configuration. This will allow the test suites to distinguish a
local osx configuration from a "remote" one.

rdar://167591463

GitOrigin-RevId: 305c413c92acfdd1df15b26080601bc66fb86dd1