[ASan] Mark test as UNSUPPORTED for iOS simulator

coverage-fork.cpp uses `fork()` which requires additional permissions
in the iOS simulator sandbox.  We cannot use `sandbox-exec` to grant
these permissions since this is a Posix (not Darwin) test.

compiler-rt/test/asan/TestCases/Posix/coverage-fork.cpp

@@ -4,6 +4,7 @@
 // RUN: %env_asan_opts=coverage=1:verbosity=1 %run %t 2>&1 | FileCheck %s
 //
 // UNSUPPORTED: android
+// UNSUPPORTED: iossim
 //
 // Ideally a forked-subprocess should only report it's own coverage,
 // not parent's one. But trace-pc-guard currently does nothing special for fork,