[compiler-rt/darwin] Disable building sanitizers on platforms without fork(). (#83485)

The watchOS and tvOS sanitizers do not compile with publicly-available
SDKs, failing on calls such as fork() and posix_spawn(). Updating the
darwin_test_archs test program to include a call to fork() allows cmake
to exclude those platforms when compiling runtimes. This allows public
builds to enable watchOS/tvOS and compile builtins but not sanitizers.