[orc-rt] Add ORC_ENABLE_OSX to control whether to build the orc runtime (#75536)

Embedded Darwin platforms have generalized COMPILER_RT_ENABLE_<PLATFORM>
configuration settings, but currently 'osx' is always eabled on Darwin.
Add ORC_ENABLE_OSX to allow explicitly *disabling* the orc runtime for
macOS platform. This can be useful if you only want to build a specific
embedded platform. It would be nice to generalize this to handle other
compiler-rt projects (i.e. add COMPILER_RT_ENABLE_OSX), but would
require additional attention from each compiler-rt project.

Note: some tests currently only are configured for osx, so these are
disabled when osx is disabled.